Minor update the pyEXP CoefStruct interface

[The9Cat]

Summary

This commit overloads CoefStruct::setCoefs() to have two call signatures:

1. The original version that returns a reference to the CoefStruct data store
2. A void version that takes as input the flattened array of the same form as CoefStruct::getCoefs() or CoefStruct::setCoefs(); that is, with the signature void CoefStruct::getCoefs(Eigen::VectorXcd& array) and sets the underlying data store from array.

This gets around the confusion of needing to set the values of C from C=CoefStruct::setCoefs() element by element which is a bad use of the numpy API, imho.

Tests

Tested on the 'custom force' notebook from pyEXP-examples/Tutorials/Orbits to confirm that both methods give the same results. This improves the readability the force function in this notebook very nicely! I will check that into EXP-code/pyEXP-examples/#13 if we decide to keep this change.

Notes

This PR adds new minor functionality and, therefore, this addition should not conflict with any current usage pattern and break existing code.

[michael-petersen]

I've been testing this; I really like the new Pythonic interface!

Also good catch on pcaeof.

[The9Cat]

Thanks for testing. Yes, hit the 'pcaeof' in doing the halo analysis for dipole. Just an omission...