UseSubringGens -- Flag for using the subring generators when checking a subalgebra basis

When isSAGBI is supplied a SAGBIBasis or a subring, there are two generating sets which can be used: either the generators of a subring or the subalgebra generators of a SAGBIBasis (which may be stored in the cache of a subring).

If the flag is set to false, then the check is applied to the subalgebra generators of $SB$. If the flag is set to true, then the check is applied to the generators of the subring of $SB$.

Functions with optional argument named UseSubringGens:

• forceSB(SAGBIBasis,UseSubringGens=>...)
• forceSB(Subring,UseSubringGens=>...)
• isSAGBI(List,UseSubringGens=>...)
• isSAGBI(Matrix,UseSubringGens=>...)
• isSAGBI(SAGBIBasis,UseSubringGens=>...)
• isSAGBI(Subring,UseSubringGens=>...)