This saves needing to do, e.g., space.ABCD(1, 2) and instead allows just space.abcd
(particularly useful in this case as all couplings are equivalent for Spaces).

Each abcd property of the relevant Connector classes returns a copy of the numeric
ABCD matrix for the corresponding coupling. This should be a preferred way of (the user)
accessing these matrices.

Also improved the docstrings for Connector ABCD methods in this commit.