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
Merge branch 'master' into modgcd_funcfield

Conflicts:
	doc/src/modules/polys/internals.rst
	sympy/polys/tests/test_modulargcd.py
Merge pull request #2365 from katjasophie/modgcd_funcfield

Modular GCD for polynomials over algebraic number fields