Optional arguments from input data sets -> **kwargs

[RobertPincus]

As noted in #225 the current code looks for optional DataArrays in the input data sets. For the gas optics this includes total_solar_irradiance; for the solvers it includes incident_flux (diffuse).

Seems like these would be better passed as DataArrays/Datasets as optional arguments and xr.merged with the problem dataset.

[RobertPincus]

Maybe the solar variability parameter(s) should persist, like in the Fortran. Use @property?

[RobertPincus]

rte.solve() should eventually take optional argument nmus to allow for higher-order quadrature in LW problems.