Truncations, GradedPieces, Dual, CombinatorialHom

Implemented methods for truncation and taking graded pieces of graded
relative ring elements.

It is now possible to compute the dual of an arbitrary graded relative
ring element.

CombinatorialHom is now available for relative graded ring elements.
HomDimension

Implement methods for computing the k-dimension of the
homset between two graded relative elements.
Installed CommonCharacters
Merge pull request #11 from sebastianpos/master

Combinatorics