Fix dependence on numbers

A declaration of the type a::Depends(0) will introduce dependence on all rationals (as the overall constant is always ignored).

Edit: can be circumvented by using a notation with \partial which mentions the coordinate name, i.e.

\partial_{x^{0}}{a} + \partial_{x^{1}}{a};

(see test 37 in tests/fieldtheory.cdb)