added GMSH_EXECUTABLE in feelconfig.h
update example
Fixes #269: split files into subfiles stored in detail/
 - make it a bit more robust and more compact
 - allow for almost automatic differentiation by enabling the `diff` function
see example nlda.cpp for an illustration
Updates #269: add an example to illustrate the latest changes in Ginac
revert changes
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop