Modal expansion: added KL, RBF and Modal Coefficients, hopefully without break everything

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.56317% with 62 lines in your changes missing coverage. Please review.

Project coverage is 75.72%. Comparing base (8366bf6) to head (9e76262).

Files	Patch %	Lines
arte/utils/karhunen_loeve_generator.py	75.23%	26 Missing :warning:
test/utils/modal_decomposer_test.py	85.41%	14 Missing :warning:
arte/utils/rbf_generator.py	85.71%	10 Missing :warning:
arte/types/modal_coefficients.py	91.89%	3 Missing :warning:
arte/utils/modal_decomposer.py	84.21%	3 Missing :warning:
arte/utils/base_modal_decomposer.py	98.00%	2 Missing :warning:
arte/utils/zernike_decomposer.py	92.85%	1 Missing :warning:
test/types/modal_coefficients_test.py	97.61%	1 Missing :warning:
test/utils/karhunen_loeve_generator_test.py	97.43%	1 Missing :warning:
test/utils/rbf_generator_test.py	98.41%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #37 +/- ## ========================================== + Coverage 75.09% 75.72% +0.62% ========================================== Files 172 182 +10 Lines 11160 11729 +569 ========================================== + Hits 8381 8882 +501 - Misses 2779 2847 +68 ```

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[alfiopuglisi]

If I understand correctly, the idea is to generalize the ModalDecomposer class to allow different modal bases (Zernikes, KL, etc). The idea is sound, but it looks to me like the implementation is done backwards: there should be a generic ModalDecomposer class with the interface, and then either:

• (inheritance) derive classes like ZernikeModalDecomposer, KarhunenLoeveModalDecomposer, etc implementing the details; or
• (composition) instantiate the desired mode generator (e.g. ZernikeModeGenerator) and use it to initialize the generic ModalDecomposer.

I prefer the composition approach, and the generator classes are already there it seems.

[alfiopuglisi]

Reorganized ModalDecomposer to have a base class BaseModalDecomposer with all the rec. generating and projection code, independent from the modal base, and a series of generators for Zernike, KL and RBF modes. Small classes ZernikeModalDecomposer, KLModalDecomposer and RFBModalDecomposer are very small classes derived from BaseModalDecomposer and only adding the modal generator instantiation.

The old ModalDecomposer has been kept without any interface changes, for backward compatibility.

[alfiopuglisi]

@lbusoni please review (your requested changes are blocking the pull request)