add BasisFunctions class
not used yet
update #279 :
 - move classes in expr.hpp into their own headers
 - document some of the functions
ignore some files
update #158
 - now lambda expression support up to 3 arguments, more is easy to do thanks to
 variadic templates
 - updated test_lambda.cpp
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop