Do not harcode PPL_Coefficient (?)

[videlec]

moved from gitlab issue #14

PPL supports computations with various coefficient types but this is hardcoded at PPL compilation time in typedef PPL_COEFFICIENT_TYPE Parma_Polyhedra_Library::Coefficient. The most standard (and default) configuration option for PPL turns out to be GMP integers.

The pplpy library assumes that the coefficient type is mpz as can be seen in the Cython declaration in ppl_decl.pxd

ctypedef mpz_class PPL_Coefficient  "Parma_Polyhedra_Library::Coefficient"

We might want pplpy to support other Checked_Number class and this should just be a matter of figuring out what compilation option has been used for PPL.