various cleanup for #490
 - remove unnecessary data structure
 - use unordered map
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop

Conflicts:
	feel/feeldiscr/doftable.hpp
add fem model (benchmark grepl)
@cdaversin: seems like they have a bug in the paper
the right hand side is most likely sin-sin and not sin-cos when looking at the figures