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!   
Before your changes, `min_` and `max_` have been expected to be numbers, so typically of type `int` or `float`.

With your changes, they should be some AstroPy quantity connected with a senseful unit.