bug fixes, rewrote Subgroups3.pg

UMass Abstract algebra bug fixes from Dan Nichols and Siman Wong.

Minor updates and bug fixes to the following problems: Congruences 8 and 12, Cosets 4,5,8,11,14, Fields 6, Permutations 6, QuotientsProducts 4 and 8, RingsIdealsHomomorphisms 3, RingsQuotientsPolynomials 3 and 4.

I also rewrote Subgroups 3.