Review astropy.units performance tips

See https://astropy.readthedocs.io/en/latest/units/index.html#performance-tips

• Use of composite units and << to avoid copies
• Replace q.to(unit).value by q.to_value(unit), see http://docs.astropy.org/en/stable/units/quantity.html#converting-to-different-units

[astrojuanlu]

Another cheap improvement:

$ grep -E 'to\(u.(.*)\).value' src/ -r | wc -l
158

Replace those by q.to_value(unit), see above.

[jonpsy]

@priyanshurohilla or anyone in that matter, Why not try in your computer the changes I made as suggested in #857 ? Maybe it’s an intrinsic problem of my computer.Let’s get that cleared, then we can get to the root of it.