NOTES:
CoCoA-4 has an intrinsic limitation on multigradings (Weights Modifier)
which does not allow zero-entries in the first row of the defining
matrix.
This function performs all the appropriate conversions for computing the
HilbertSeries wrt any positive grading (IsPositiveGrading).
Example
Use R ::= QQ[x,y,z];
WM := Mat([[1,0,0],[1,-1,0]]);
HilbertSeriesMultiDeg(R/Ideal(Indets())^2, WM);