Add relative position to the sun.

[jeenriquez]

Describe the solution you'd like
Given the RA&Decl and time of the observation, calculate the angular distance to the sun (particularly if the observation is during the day).

Additional context
This is an important to know to calculate the scintillation effects caused by the IPM.

[telegraphic]

There is some code in #34, and some code in http://docs.astropy.org/en/stable/coordinates/solarsystem.html that could be used as a base to do this in astropy.

I would like to try and do all calculations with astropy, and get rid of pyslalib and pyephem if possible (although this is probably easier in pyephem!)

Who wants to take the lead on this? (I'm pretty busy atm)

For development, can we write this as a function that takes a blimpy.Waterfall object as its main argument? E.g. https://github.com/telegraphic/blimpy/blob/master/blimpy/ephemeris/compute_lst.py

[jeenriquez]

Agree on astropy, and the type of function we want.
Also, no time from me atm. :)