fix: memory leak while calling gcd with a term and a non-term poly
new: division and gcd of integer polynomials via factory

added conversion functions between integer polynomials in Singular and Factory
extended the singclap_gcd and singclap_pdivide functions to cover the case where
ground ring is integer
Merge pull request #619 from YueRen/gcd_fixes

Gcd fixes (gcd for poly over Z)