matrices: Documentation and LU implementation simplification
matrices: Documentation and LU implementation simplification

Added documentation to LUdecomposition() LUdecomposition_Simple().
Changed pivot search in LUdecomposition_Simple() to prefer
candidate pivots that iszerofunc can guarantee are zero over those
that iszerofunc returns None for.

Added deprecation warning when keyword argument  rankcheck is False.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy