add and update some doxygen documentation
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop
add documentation to the function expand (fixes #391)
Merge branch 'develop' of https://github.com/feelpp/feelpp into feature/interpolate
Merge branch 'feature/interpolate' of https://github.com/feelpp/feelpp into feature/interpolate
up #392
 - homogeneize tangent (only in 2D) as for the normal
 - fix interpolate in Nedelec
 - now T() returns the unit Tangent to the edge in 2D