Support general matrices in `IsMatrixObj`.

[ThomasBreuer]

Eventually it would be good if the code of the Forms package would support also matrices that are not lists of lists. Chapter Vector and Matrix Objects of the GAP Reference Manual describes the ideas and the rules.

For example, one cannot access the i-th row of such a matrix M via M[i] (but this operation is not really important in the context of the Forms package), and one should access the entry in row i and column j via M[i,j] not via M[i][j].

Typically, the matrices in the filter IsMatrixObj store an attribute BaseDomain, whose value is the domain over which the matrix in question is defined. If one uses this attribute instead of the field generated by the matrix entries (and if this becomes documented) then the one-argument versions of BilinearFormByMatrix. HermitianFormByMatrix, etc. will become more useful.