Addresses a number of issues raised in the PR:

   - Support any angular unit in the unit system (including, e.g. arcmin and arcsec), not just hours, degrees and radians

   - Clarify the units of the 'bounds' argument

   - Allow for passing a unit in both `value` and `unit`, in which case `unit` takes precedence.

   - Remove plural names for units, including "radians", "degrees" and "hours"

   - Make Quantity(other_quantity) work
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