Simplify context managers for enabling units.
Fix doc warning
Trial with equivalency=(u.rad,None)
Add radian_is_dimensionless function to equivalencies.py
Add test cases, and, as a result, correct is_equivalent
Also rename equivalency to angles_dimensionless
Add documentation for angles_dimensionless in units/equivalencies.rst
Correct equivalency tests
Use same test for (unit, None) equivalency in is_equivalent and _apply_equivalencies
More reliable test for (unit, None) converter
Change build-in equivalency test to one that uses a class
This since objects can no longer be initialised with equivalencies
Change name to dimensionless_angles
Remove xfail equivalency context manager test given renaming in #1662
Add documentation for angles_dimensionless in units/equivalencies.rst
Add equivalencies to UnitRegistry, as well as add/set/context managers
Add documentation for the equivalency context manager
Use equivalencies=None, so that empty list can override global default
Cleanup for equivalencies=None; further tests
Adapt doctest to deal with small representation errors
Add comment to decompose to ensure no equivalencies are ever passed
Reinterpret equivalencies=[] and None; normalize on UnitRegistry input