Beta plot for python nnls

[maindlt]

Plotter.beta_plot(...) now uses the existing LegacyOrbitLibrary.get_model_intrinsic_moment_constructor() method and does not rely on the LegacyWeightSolver output any more.

get_model_intrinsic_moment_constructor() calls the rather time-consuming orbit library method read_orbit_intrinsic_moments(...) every time it is called. This decreases performance, especially when several models in the beta plot share the same orbit library. Therefore, read_orbit_intrinsic_moments(...) can now cache the calculated intrinsic moments in files (switched on by default via the cache=True method parameter). In the test script (see below) this reduces the time required for creating the beta plot from 10 seconds to about 1 second.

To test: please run the temporary test script dev_test/test_beta_plot.py which works off the existing user_test_config_ml.yaml configuration file:

• The weight_solver settings type: "LegacyWeightSolver" and nnls_solver: 1 should produce 4 plots in the plot directory: newanisotropy_var.png, newbetaz_var.png (both using the new implementation) and oldanisotropy_var.png, oldbetaz_var.png (using the old implementation, relying on LegacyWeightSolver output). They should be the same.
• The weight_solver settings type: "NNLS" and nnls_solver: 'scipy' should produce 2 plots in the plot directory: newanisotropy_var.png, newbetaz_var.png (both using the new implementation) and then crash (because the LegacyWeightSolver output isn't there). These plots should correspond to the ones above.

Before merging:

• Remove the test script dev_test/test_beta_plot.py
• Remove the test implementation in plotter.py (annotated with # TODO before merging).

Closes #339.

[prashjet]

I ran the proposed tests and got the expected outcome - great, many thanks Thomas! Will leave the final TODO-s and merging to yo.