mjo/clan/t_algebra_clan.py: speed up ComplexHermitianClan

mjo/clan/t_algebra_clan.py: speed up ComplexHermitianClan

To speed up ComplexHermitianClan, we use a quadratic field extension
for the entries of the ambient matrix space rather than (basically)
assuming QQ and QQbar. But before we can do that, we need to adjust
from_matrix() to reject mismatched entry algebras. We also need to
special case several real/complex field pairs, because the quadratic
extension procedure does not always give us something usable -- our
Hurwitz matrix algebras are finicky.