CategoryConstructor should support installing functions for arbitrary CAP operations

[zickgraf]

Currently, CategoryConstructor only supports installing functions for some return types but not all of them. For example, object, list_of_objects and morphism are supported, but list_of_morphisms is not supported because it is unclear how the source and range of the morphisms in the list would be specified. For example, this affects BasisOfExternalHom. It would be nice if one could simply give a custom function string for BasisOfExternalHom. This custom function would have to handle the returned list of morphisms itself, but could at least make use of the functionality independent of the return type, e.g. underlying_arguments.

This in turn would allow DummyCategory to install arbitrary operations.