Adding dayside/nightside integrated temperature convenience method

[bmorris3]

The code in this PR implements a feature that both @KathrynJones1 and @mhooton have requested: integrated dayside/nightside temperatures from the model parameters.

This is implemented with a convenience method on the Model object which gets called like this:

from kelp import Planet, Model, Filter

f = 0.68
C_ml = [[0], 
        [0, 0.2, 0]]
p = Planet.from_name('HD 189733')
filt = Filter.from_name('IRAC 1')
m = Model(-0.8, 0.575, 4.5, 0, C_ml, 1, planet=p, filt=filt)

dayside, nightside = m.integrated_temperatures(f=f)

where dayside, nightside are the integrated dayside/nightside temperatures in units of Kelvin.

[bmorris3]

These tests are failing because of sphinx-doc/sphinx#8880, pinning sphinx version.