Algebras meta issues

[thofma]

Rainy day todo list for algebras:

• Add hom(A, B, ::Vector) constructor.
• Morphisms defined by hom(A, B, data) do not check whether they are morphisms.
• Think about antihomomorphisms (separate type)? Example is $A \mapsto A^t$ on matrix algebras or $g \mapsto g^{-1}$ for group algberas.