gf: move closest_pt in gf

- closest_pt now in gf.
- added a new trait to make that computation
- det_manip : correction to foreach. Tested in ctseg.
parameters: add a name into the opaque object

- for clearer errors message, add a name for the object
- this is just for errors.
parameters, py : add declaration of defaults

- for usage in cython
- add in the defaults the type of the object
- add a defaults.cpp for compilation speed
- parameter: I now use the h5_name as a type_name, not the demangled
  one, so that I can use it in printing help functions.