Initial code for the new evaluation methods, so far concerning only monomial.
piranha.hpp: a minor bit of internal doc.
k_monomial: beautify the eval_type definition.
monomial: add an assertion.
pyranha: add a bit of internal doc.
k_monomial: port to the new evaluation method.
rtkm: beautify the eval type definition, and add a piece of internal doc.
rtkm: initial commit for the new evaluation method, not tested or documented yet.
rtkm: finish the work on the new eval method, plus a couple of test additions.
series: adapt the evaluation method for the new key methods, move evaluation tests into series_02, and fix a missing environment creation there.
math: switch the order of template params in math::evaluate(), so we can use the explicit template syntax of the series evaluation method.

Also rework a potentially problematic syntax in a test.
Change the default evaluation implementation to return a copy of the input argument.

This allows us to get rid of a few implementation specs. Test changed to reflect the new behaviour.
pyranha: implement, test and document the evaluate() math wrapper.