Unclear preconditions for induceSpanMorphism

[ggazzi]

The method induceSpanMorphism of the FindMorphism class will sometimes generate an invalid morphism, since the desired morphism doesn't always exist.

This should be documented, or even better, it should return Nothing when the morphism doesn't exist.

[lm-rodrigues]

Maybe this function should be moved from FindMorphisms, since it does no implement the "intuitive" behaviour of other functions in that Type Class.