automatically translate pure python models to C

[pkienzle]

As an exploratory project we created a translator that converts pure python models directly to C, allowing them to run on the GPU. See for example _cylpy, the cylinder model in python.

TODO: update example to define F(q) rather than I(q)

For now this is only enabled if py2c = True is in the model file. The impact on the rest of the code is minimal, so including this shouldn't break anything.

Long term it would be nice to have only C models so that we do not need separate code paths for python models and C models. The python model infrastructure has already fallen behind, and doesn't support the changes for jitter or magnetism. Future changes will become increasingly difficult.

On the other hand, updating the translation code to production quality and maintaining it will also require work. It is not extensively tested, and it only supports a small subset of python. It is good enough for fortran-style code, but idiomatic python and numpy code will quickly trip it up.

I will leave it to others to decide if it will be easier to teach the SAS community to write models in C than it will be to teach the community to write models in restricted python AND maintain the restricted python to C converter.

Note that there are several projects that do a better job of translating python to C (nuitka, pypy, cython and numba for example), but I don't know if we can get any of them to produce code that we can run in OpenCL.

apply after beta approx branch

[pkienzle]

Updated to use the beta_approx branch

[wpotrzebowski]

As agreed on the call it will be moved to discussions

[wpotrzebowski]

Closing this PR and archiving in discussion #559