More linear algebra over fields work, part n.
fixed: clean, norm now work for matrices, mutable matrices and poly rings

over RR, CC fields.
LU decomp for RR,CC now go through mat-DU-template.
Fix a bug introduced in commit e4d1989.

The problem: polynomial rings over RR or CC also have precision, but
the standard division doesn’t work.  In fact, over poly rings, the
LU routines should NOT be used anyway…