Documentation + generalization improvements

[bmorris3]

Based on work with @KathrynJones1, there are several docs improvements that we can make, in addition to some generalizations we can consider.

Here we can keep a running list of which improvements which can be made soon, and which have been addressed.

• either write an input checker or state clearly in docs that theta, phi are assumed to be monotonically increasing before passed to the temperature_map function and each of the cython functions
• clearly put up caveat notes in the docs that this model is for thermal emission only, remove old "reflected" option
• put up a troubleshooting note in the docs that describes how to get kelp working on Mac OS X Catalina (downgrade to command line tools v11.5)
• add other missions to filters.json file (Kepler, CHEOPS)
• implement a theano-compatible version of the cython scripts for use with the exoplanet package
• document how to use Hermite polynomial recurrence relations to arrive at the form of the spherical harmonic terms used in kelp
• add f (Greenhouse) parameter to the tutorials in the docs
• add support for simultaneous fitting in multiple bandpasses
• specify radians/degrees for hotspot offset parameter in docs
• in getting started, mention background on specific values of alpha and omega
• adjust title in the last bullet point in the homepage