Fixes #491.
Really fixes #491.

opInterpolation matrices were never assembled and were left open prior to this commit.
Backends may now compute x = A^T y.
Now works with both PETSc and Eigen backends.
Hacking around for better performance (2x speedup)
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
@cdavesin minor up
Update .travis.yml

allow use of new travis-ci infrastructure (4GB + 2 cores) (before 3GB + 1.5 cores)
Update .travis.yml

revert (we need sudo)
up #277 : fusion2d
fix testsuite compilation part I
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop
up #277 : clean obselete code
up #277 : add others tests
fix testsuite compilation part II
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop
fix testsuite compilation/execution part III