Initial implementation of FanPT in PyCI #70

This initial implementation is based on the original FanPT code written by Ramon's Group to the pilot code Fanpy. Here is a brief description of the code:

pyci/fanci/fanpt_wrapper.py

Contains the FanPT solver (solve_fanpt) function and additional functions.

pyci/fanpt/base_fanpt_container.py

Defines general FanPT class (FANPTContainer). It is used by derived classes FANPTContainerEParam and FANPTContainerEFree.

pyci/fanpt/fanpt_cont_e_param.py

Define a FANPTContainerEParam class, which contains the data and methods required for a FanPT calculation using energy as a parameter.

pyci/fanpt/fanpt_cont_e_free.py

Define a FANPTContainerEFree class containing the data and methods required for an energy-free FanPT calculation. It is derived from FANPTContainerEParam.

pyci/fanpt/fanpt_constant_terms.py

Define the FANPTConstantTerms class, which contains the constant terms of the FanPY system of equations and methods.

pyci/fanpt/fanpt_updater.py

Contains the FANPTUpdater class, which is used by the FanPT solver to update FanCI parameters and energy.

pyci/test/test_fanpt.py

Includes two tests for FanPT solver, using Energy Free and Energy as Parameter flavors.

There's room for improvement in code design and file organization, especially in matching the PyCI interface for seamless integration.
Let me know if you have suggestions, and I will work on them.

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@msricher and @marco-2023 can you look at this and see what the next step should be? I have a feeling some of the data classes may be better off as named tuples.

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I've looked at it and I think this is fine the way it is. The next step might be making sure that pCCD+S can now be reliably solved, so that I can do a better test of the stochastic optimization?

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Ok, sounds good. Thanks for looking at it @msricher .

I agree with your next step.

@carlosevmoura , we'll do a formal code review on it (I haven't seen the pull request but I'm way behind). That will usually lead to some changes. But then we will have a functional version. I anticipate a refactor at some point of a lot of FanCI stuff, but I am too overwhelmed to think about that for now.

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