Cooper pair box

[superplay1]

What

It adds the class CooperPairBox model to chip.py

Why

Additional functionality for regions where code of Fluxonium model fails.

How

I calculate the Hamiltonian in charge basis a-nd diagonalize to be again in energy basis. The transformation to the energy basis is done for the initial ng-value.

Remarks

I noticed that sometimes tf.range() does not what it should when the used dimensions size is big.

Regarding the checklist below
-I don't know how to write tests properly.
-I am not sure if a notebook is really necessary. You could just take the two_qubit_example and use CooperPairBox instead of Transmon except that I am not sure If coupling will work in the same way.
-The Documentation is generated automatically, isn't it? If not I don't know how to update it.
-Don't know how to add something the Upcoming Release section

Checklist

Please include and complete the following checklist. Your Pull Request is (in most cases) not ready for review until the following have been completed. You can create a draft PR while you are still completing the checklist. Check the Contribution Guidelines for more details. You can mark an item as complete with the - [x] prefix

• Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• Formatting & Linting - black and flake8 have been used to ensure styling guidelines are met
• Type Annotations - All new code has been type annotated in the function signatures using type hints
• Docstrings - Docstrings have been provided for functions in the numpydoc style
• Documentation - The tutorial style documentation has been updated to explain changes & new features
• Notebooks - Example notebooks have been updated to incorporate changes and new features
• Changelog - A short note on this PR has been added to the Upcoming Release section

[codecov]

Codecov Report

Merging #184 (a1aa398) into dev (bd06408) will decrease coverage by 0.63%.
The diff coverage is 12.06%.

❗ Current head a1aa398 differs from pull request most recent head 1d685e5. Consider uploading reports for the commit 1d685e5 to get more accurate results

@@            Coverage Diff             @@
##              dev     #184      +/-   ##
==========================================
- Coverage   73.24%   72.61%   -0.64%     
==========================================
  Files          37       37              
  Lines        5561     5619      +58     
==========================================
+ Hits         4073     4080       +7     
- Misses       1488     1539      +51     

Impacted Files	Coverage Δ
c3/libraries/chip.py	53.20% <12.06%> (-4.47%)	⬇️

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[lgtm-com]

This pull request introduces 1 alert when merging 3c7588a into bd06408 - view on LGTM.com

new alerts:

• 1 for Variable defined multiple times

[lazyoracle]

Re: Tests

The contributing guide has a section on tests to help onboard new contributors to writing tests for their code. The basic idea is to convert one of the scripts that you are using for checking your code into the structure for a test script following the pattern seen in the other tests in our repo. It also includes a link to resources to better understand how to write tests in python. Can you please go through them and let us know if they are not adequate/useful enough to help you write tests for this PR? We can use this feedback to improve the onboarding document.

[nwittler]

This will throw an error when "offset_amp" is not in the Envelope parameters. An easy fix would be to add it in the default here: https://github.com/superplay1/c3/blob/1d685e5d8daf73640898040ee5b3ab8bcce02695/c3/signal/pulse.py#L47-L54