update issue #239: refactor polynomialset.hpp
 - moved `update()` function to polynomialset_impl.hpp
 - do not inline `update()`
update #251 and #252
 - introduce base clases for hdiv and hcurl polynomial set
 - introduce is_hdiv_conforming<> and is_hcurl_conforming<> to check whether a
 polynomial is hdiv or hcurl conforming repectively
 - implement 1st (covariant) Piola transform for hdiv conforming elements
 - change sign of basis function based on the orientation of the edge (2d for
 now) in order to ensure continuity of the normal component
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop