Add normal epimorphisms and normal monomorphisms

[ScriptRaccoon]

This PR implements another part of #267 by adding normal epimorphisms and normal monomorphisms. These are only defined in categories with zero morphisms.

Their basic properties are added (every normal epi is regular, etc.), and also examples showing that some implications are not true:

• the embedding $A_3 \hookrightarrow S_3$ in Grp is an example of a normal monomorphism that does not split
• the projection $Z \to Z/p$ in Ab is an example of a normal epimorphism that does not split
• the multiplication $2 : Z \to Z$ in TorsFreeAb is an example of a monomorphism (and epimorphism) in an additive category that is not normal

Because of the two new properties, the identity morphism id_X in Set (for a set X) does not have all properties of morphisms anymore. Therefore, also the identity morphism id_G in Grp (for a group G) has been added. It has all current properties. This is also verified in the test script.