Moving accumulated changes from branch develop onto master.
*) Bug fixes: rescaling LW calculation to treat scattering
*) Sensitivity (Jacobian) of LW fluxes to surface temperature
*) NRLSSI2 model of solar variability including the ability to specify total solar irradiance (#43)
*) Parameterization for optimal single angle at which to do LW no-scattering calculations

Argument lists for ty_gas_optics_rrtmgp%load() are changed for both LW and SW versions.

The optimal angle works but has no effect; updated data to come.

Testing and continuous integration has also been expanded, including a new set of regression tests in tests/ 

Co-authored-by: Valentin Clement (バレンタイン クレメン) <clementval@gmail.com>
Co-authored-by: Benjamin R. Hillman <bhillma@sandia.gov>
Co-authored-by: Dmitry Alexeev <53860485+alexeedm@users.noreply.github.com>
Co-authored-by: inpolonsky <51209434+inpolonsky@users.noreply.github.com>
Co-authored-by: jdelamere <jdelamere@users.noreply.github.com>