Separate and use model and lens parameters for datagen.py

[hgeorgsch]

This includes cleaning up the C++ code. We should probably have a --lens option for the lens model, and a --sampled flag to choose between direct use and sampling.

[hgeorgsch]

Review this, and close #111 in the process.

[hgeorgsch]

ongoing on branch refactor/split-model-lens

This may be done; testing remains.

[hgeorgsch]

There is a problem with the Sampled Raytrace Model, but it is worse on the develop branch where it segfaults.

[hgeorgsch]

Seems ok; merged.