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!   
Have you check which functions were using `to_angstrom()`?
```shell
$ find ./ -iname '*py' -exec grep -Hn 'to_angstrom' {} \;
./net/vso/attrs.py:26:from sunpy.util import to_angstrom
./net/vso/attrs.py:60:            to_angstrom(v, waveunit) for v in [float(wavemin), float(wavemax)]
./net/vso/attrs.py:322:        attr.min <= to_angstrom(float(it.wave.wavemax), it.wave.waveunit)
./net/vso/attrs.py:326:        attr.max >= to_angstrom(float(it.wave.wavemin), it.wave.waveunit)
```
I've found these occurrences that were not taking into account in your branch.