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 would suggest to don't rename `assert_array_almost_equal` as `a` but change maybe the input to a tuple or dictionary:
```python
inputs = {'ddays': 10, 'latitude': Latitude(30, u.deg)}
assert_array_almost_equal(diff_rot(**inputs), 136.8216 * u.deg, decimal=4)
```