Factors renaming

Continuing with the renamings in wolf, the naming of factors is still an issue. We have 3 types of differentiation:

• Analytic
• Automatic
• Numeric I think that we don't have any numeric factor. We don't have a pure virtual class for that.

The naming of the factors should contain from which class they inherit from (following wolf naming rules): factor_{aut/ana}_xxx.h

Since "analytic" and "autodiff" is too long, I propose to put just 3 letters (or two) because with 4 we would have anal factors and it could be misleading XD

What does the naming guy (@jsola) think about this?