Xallarap - after merge TO DO list

[rpoleski]

Some useful changes to be done after xal branch is merged:

• Example 16 - add xallarap parameters to _set_default_parameters() and _check_fixed_parameters()
• 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
• Left/right hand system - see eq. 11 in the EXOFAST 2.0 paper
• consider adding sqrt(e)*sin(omega) and sqrt(e)*cos(omega) parametersization (ref - p.3 right column)