Some useful changes to be done after xal branch is merged:
[x] Example 16 - add xallarap parameters to _set_default_parameters() and _check_fixed_parameters()
[x] change ModelParameters._check_valid_combination_1_source_standard() so that it would be useful for Keplerian lens motion (i.e., remove xi_ and run tests on that).
[ ] Add on-sky angles (RA, Dec) calculations (requested by @jenniferyee; also could be useful for RV follow-up).
[ ] Example with calculations of Kepler III law.
[ ] Warning for stupid values of angles etc. - REMOVE
Some useful changes to be done after xal branch is merged:
_set_default_parameters()and_check_fixed_parameters()ModelParameters._check_valid_combination_1_source_standard()so that it would be useful for Keplerian lens motion (i.e., removexi_and run tests on that).sqrt(e)*sin(omega)andsqrt(e)*cos(omega)parametersization (ref - p.3 right column)