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!   
I'm not sure we want to make the query always in Angstroms... But in any case I would change the `wave_unit_catcher()` so it returns a `Quantity`. That way we would just use here `avg_wave_len`