rename _coerce_2 to _to_common_subfield; cosmetic changes
tiny optimization for _to_common_subfield and implementation of
is_finite.
fix comparison of pseudo-Conway polynomial trees
add a _latex_ method to the elements
rename/fix change_level(); new doctests
matrix2.pyx: no special case for finite fields in F.algebraic_closure()
Experimental version of Fp-bar from trace formulas