Allow symbolic grades dim-k in wedge products

[duetosymmetry]

As reported by Alejandro Jiménez Cano in this thread. In Sec. 2.6 of xTerior.nb, in the cell below "We still need definition when acting on Times", we use Positive[] to check whether terms are scalars (0-forms). Unfortunately Positive stays unevaluated if it acts on e.g. Positive[dim-2], so such terms are not detected. Do something more robust.

[wtbgagoa]

Addressed in #5