Handle symbolic degree dim-k when canonicalizing Wedge products

[duetosymmetry]

As reported by Alejandro Jiménez Cano in this thread. Haven't had a chance to think about how to allow this.

[wtbgagoa]

This is an issue about the way xTensor deals with graded algebras. Apparently, it is not designed to work with symbolic grades. For the example of the thread, the following workaround seems to solve the problem (I didn't fully tested it yet) .

(* Definition of forms with symbolic grade *)
xAct`xTensor`Private`checkgrade[Wedge -> dimM - 1] := Null
DefDiffForm[dfC[], M, dimM - 1, PrintAs -> "C"]  

(* Canonicalization of expressions containing objects with symbolic grade *)
xAct`xTensor`Private`ToObject[
  xAct`xTensor`Private`AddedSign[(-1)^(dimM), expr_]] := 
 MapAt[xAct`xTensor`Private`VerbatimProduct[Times][(-1)^(dimM), #1] &,
   xAct`xTensor`Private`ToObject[expr], 1]

(* After executing the previous code the following gives the expected result *)
Hodge[metricg][dfA0[a]]\[Wedge]dfB1[b]
% // ToCanonical

I will post a comment about this in the xAct group after doing some more tests.