DOC: Improve docstring.
TST: Use numpy.testing.assert_raises decorator
Fix silly failure.
STY: Break up exception (@ElDeveloper)
BUG: PCoA should also be easily importable :)
Accept a SymmetricDistanceMatrix as input in PCoA.
Added meaning of ddof.

It's the usual name for this in std/var computations in numpy.
Remove logic from return stm, and unnecessary return.
Improve acceptance of SymmetricDistanceMatrix.

The change introduced in 7e03d06751cc70d6c77885c36b8e94575df3f17e
was very dangerous because passing a numpy array by mistake (which also
has the .data attribute) resulted in cryptic errors.
TST: Add tests for PCoA.
STY: Break long line
STY: Break another long line.