The code would previously only consider the last chain mapping in
consideration. As a result we find better scores in tests.

Raise a ValueError upon initialization.

Fixes the SDF reader and documents the convention.

Reason is, it's unclear if they are useful anywhere else

Qualified name value is not useful. Plus they are redundant with
the details.

Loading networkX module is hopefully enough

This fixes substructure matches

Fixed elsewhere by b6270ff1

Nonsense results in that context means that the model does not
cover the desired substructure in target.