Integration of astropy.units in sunpy codebase

Attempt to integrate the astropy.units into sunpy. Every function that were taking or giving a quantity now must have astropy quantity associated with them. Please tell me if I have made mistakes anywhere, or if it could be done in better way, I will make changes! 
Thank you!   
Now that https://github.com/astropy/astropy/pull/2637 has been merged, I think we can get rid of the whole file :)

I would say that `to_angstrom()` function is useless when we have all under `Quantities`.