Refactor processing for the New-PI credit

[QuanMPhm]

Dependant on #108, the implementation of the New-PI credit should be implemented as a Processor subclass.

[QuanMPhm]

@knikolla @naved001 While writing the PR for this issue, it occurred to me that the New-PI Processor will be dependant on the ValidateBillablePIsProcessor proposed in #108. This is because we would want to know which projects or PIs are non-billable before applying the New-PI discount.

As such, the New-PI Processor has a dependancy (ValidateBillablePIsProcessor), it also follows that BillableInvoice has a dependancy as well (ValidateBillablePIsProcessor) because it reads a field (Is Billable) provided by this processor.

For the purpose of this issue, this only means that I believe this issue is dependant on #108. For the purpose of code structure, I just wanted to highlight the fact that our Invoice and Processor classes may have dependencies of one (or more) Processor, in case this is something you guys might have concerns with.

For now, I will note in the docstring of Invoice and Processor subclasses if they have any specific dependencies.