experimental: only show the last step if we have more than 2 steps (to avoid always showing the imaginary part for complex fields)

Onelab "changed" flag is now multi-level (not just true/false). This allows finer granularity for complex clients. For example, Gmsh now responds to 4 levels of "changed"

- changed == 0: nothing to do
- changed == 1: only save the mesh (useful if we only chanaged some physical groups)
- changed == 2: only remesh and save the mesh (useful if we only changed some meshing paramaters)
- changed >= 3: reload geometry, remesh and save the mesh

better GetNumber and GetString parser functions (old GetString function has been renamed GetStringValue, to match GetValue)

- removed onelab::region and onelab::function (they complexify the interface, but bring no additional value compared to onelab::string)
- removed ONELAB2 experiment
- made client/Model name persistent, and define Gmsh/Model name so that we can uncouple e.g. .geo/.pro pairs

changing the gmsh source code, maybe through a onelab command ?)

allow lists in DefineConstant + on command line with -setlistofnumbers + propagate info to onelab clients

geometry. It introduces subtle bugs when reading high-order post-processing views.

IMHO we should have an explicit command to do this, and not modify the mesh without
warning just by reading a file

The parser wanted to create extruded volume begining systematically from 1. 
Thus, when there already existed volumes with number 1,2,3,etc, extruded volumes was created.

- ParseString can now run in the current model directory (useful when parsing things with relative filenames)
- New View.Time option to select the timestep according to the time value