Bug in Observatory.get()

[paulray]

I think there is a bug in the class method Observatory.get(). It explicitly adds the new Observatory object to the registry with this line:

PINT/src/pint/observatory/__init__.py

Line 303 in b0fcc44

cls._register(obs, name)

But, it should already be added to the registry from the __init__() method, which calls the class method _register().
So I think this line should be deleted.

[paulray]

Fixed in #1763