sparse version of m4gb

[cr-marcstevens]

[cr-marcstevens]

A sparse version would just operate on non-reducable terms that have a non-zero coefficient somewhere. Currently we index all non-reducable monomials up to the e lcm of the critical pair we're processing. A sparse version would be more efficient for more sparse systems, as well as be efficient for dense systems over fewer variables as it was compiled for.