Fixed a bug in HermiteKernel.

Divison by a non-unit is not allowed. Because of recent changes
in GAP (see changes for 4.7) this now causes the output to be fail
instead of something (possibly wrong).
Changed some comment.
Corrected filters for SparseKroneckerProduct over GF(2).

Before, this method was always used and every Kronecker
product of two sparse matrices was in GF2.