Allow quantity initialisation from Unit or a string
Add u.Quantity('') and u.Quantity('m') to test cases; adjust to make first work
Introduce and use from_unit and from_string class methods
Now also use from_unit in initialiser
Move string/unit check a bit up, so one can do u.Quantity(u.pc, u.m)
Small changes following rebase to quantity-speedup
Add entry to CHANGES.rst
Add PR number to CHANGES.rst