polys: implement the modular GCD algorithm for polynomials over
algebraic fields
polys: add tests for func_field_modgcd
polys: some improvements in modulargcd

_trunc now uses PolyElement.rem() directly.
_minpoly_from_dense now uses DMP.terms() directly.
polys:  improve code style in modulargcd
polys: add documentation in modulargcd
polys: additional tests for modulargcd
polys: add documentation for modulargcd to sphinx
n*m is even if n+m is odd (old assumptions)
n*m is even if n+m is odd (new assumptions)
UnXFAIL Wester test_H3
added condition in strong pseudoprime
added ``sqrt_mod, quadratic_residue, primitive_root, nthroot_mod``
added `is_nthpow_residue``; faster ``n_order``
added case in ``nthroot_mod``
fixed failures in Python3
fixed bug in  ``_sqrt_mod1``
added new functions in ``ntheory.rst``
fixed bug in  ``_sqrt_mod1``
fixed misprint
fixed bug in ``primitive_root``; increased coverage
fixed bugs in ``primitive_root``