Add copy parameter to quantity instantiation; set it to False wherever the
value instantiated is already known to be a copy.
Also adjust __quantity_instance__ in Angle, though incomplete pending #1310
Replace repeated parameters in __quantity_instance__ with **kwargs
Ensure angles can deal with copy option
Small corrections for documention of copy option in quantity.py
Also ensure copy arguments get passed on if given to Longitude/Latitude
Correct behaviour for instantiation with (q, q.unit, copy=False); add tests
Replace .base tests (problematic on travis) with others
Make clear in documentation that copy=False should be used with care
Merge pull request #1271 from mhvk/units-quantity-copy-option

Add a copy= argument to Quantity initializer