Code from `BillableInvoice` and `DiscountInvoice` has been copied over
without change

The `DiscountProcessor` class has been created, to be subclassed by
processors which applies discounts, such as the New-PI Credit.
This processor class introduces an important class constant,
`IS_DISCOUNT_BY_NERC`, which detemines whether the final balance,
as opposed to the PI balance (more info below), reflects the discount
being applied.

The New-PI credit is now implemented by `NewPICreditProcessor`

During discussions about billing, it was made noted that some discounts
are not provided by the MGHPCC, but instead from other sources, such as
the BU subsidy, which is provided to BU PIs by BU. This provided
motivation for a `PI Balance` field, which would reflect how much money the
PI should be billed, as opposed to the `Balance` field, which currently
reflects how much money the MGHPCC should receive. These two fields would
not equal each other if the PI received discounts not provided by the
MGHPCC.

Implementation of `NewPICreditProcessor` and the new billing
feature required a range of changes:
- `apply_discount_on_project()` in `DiscountProcessor` has been slightly
modified, where the PI balance and MGHPCC balance is now calculated seperately.
- As `BillableInvoice` no longer performs any processing itself, the
dataframe from `NewPICreditProcessor` is now passed to all invoice objects.
- The test cases for the New-PI credit have been refactored. Test cases
for the new billing feature is not written yet. I plan to write
them when the processor for the BU Subsidy is implemented
- With the new processor, certain Processor and Invoice classes depend on
fields created by other Processors, such as the case with
`NewPICreditProcessor` and `ValidateBillablePIsProcessor`. As such, docstrings
have been added to indicate dependancies

the `DiscountInvoice` class is now removed. No invoice class will
now class `_prepare()` or `_process()`.

`BUSubsidyProcessor`, which handles processing for the BU subsidy,
sets `IS_DISCOUNT_BY_NERC` to `False` because the
subsidy is not provided by NERC. Because of this, `BU Balance` indicates
the money which BU (not the PI they are subsidizing) owes to the
MGHPCC.

The test cases for the BU Subsidy has been refactored to be
more robust and readable.
The `Project` field has been added to `invoice.py`

Prepayments have been implemented by a new processor, `PrepaymentProcessor`.
The full implementation for prepayments involved the following changes:
- Several new columns names have been added to `invoices/invoice.py`,
including column names for the prepay data files and for the exported
invoices
- Many invoices will now export with 4 new columns: `Prepaid Group Name`,
`Prepaid Group Institution`, `Prepaid Group Balance`, `Prepaid Group Used`
- 4 command line arguments have been added to `process_report.py`.
3 of them allows the user to pass in a local version of the prepay credits,
contacts, and projects file. The last one (`—prepay-debits`) allows passing
a local version of the prepay debits files, and defaults to fetching from s3 if not
provided
- A set of test cases have been added for `PrepaymentProcessor`

Since the implementation of this feature required a lot of logic decisions
(i.e What happens if a prepaid project is active, but has no
funds?), below is (hopefully) an exhaustive list of code logic decisions
that were made. These can also be inferred through the test cases.
- Prepay projects listed in `prepaid_projects.csv` are identified by their
project name, not project - allocation name
- Attempting to process past invoices (“backprocessing”) may result in
incorrect output due to the nature of the prepay debit ledger
- While backprocessing is not supported for past months, processing
the same invoice month twice will still return correct output. In this
case, the month’s debit entry may be will be overwritten
- Prepay balances can be used in the same month they are added.
- The time range in which prepay projects are considered “active” includes
their start and end date
- After processing of any given invoice month, debit entries for that
month will be added. I emphasize this for clarification. A debit entry
such as:

`2024-11,G1,1062.48`

Should be interpreted as:
In the period from 2024-11-01 to 2024-11-30, prepay group G1 spent $1062.48
As opposed to:
In the period from 2024-10-01 to 2024-10-31, …

- If prepay projects are “active” but their prepay group has $0 balance,
their prepay info (group name, contact email) is still included, but the
prepay balance will be displayed as $0 and the prepay used as an empty field

…r` to `util`

Since these functions are now used in more than one location (`add_insti_proc`
and `prepay_proc`), it makes sense for them to be relocated to `util`

The test case checking institution-processing has changed slightly

This invoice has custom output filepaths for S3,
and only lists projects belonging to prepay groups not
managed by MGHPCC