Move normalize_names() out of CategoryObject

[jdemeyer]

It makes no sense that normalize_names appears in the TAB-completion of all CategoryObjects. It also makes little sense that it's a method of CategoryObject.

1. Move normalize_names and _certify_names out of CategoryObject and make them non-method functions.

2. Remove the redundant parent_gens.normalize_names, parent_gens._certify_names, parent.normalize_names and sage.schemes.toric.variety.certify_names.

3. Always interpret ngens=-1 as "unknown number of generators" and ngens=0 as "no generators". The previous implementation did this inconsistently.

4. Add doctests.

Note that many more classes should probably use normalize_names. I didn't fix this here. Example from src/sage/algebras/jordan_algebra.py:

        if names is not None:
            if isinstance(names, str):
                names = names.split(',')
            names = tuple(names)

[tscrim]

Big +1.

[jdemeyer]

New commits:

​280312f

Move normalize_names to function in category_object.pyx

[tscrim]

LGTM.