10.3 Option handling

To add a new option in Gmsh:

1. create the option in the Context_T class (‘Common/Context.h’) if it's a classical option, or in the PViewOptions class (‘Post/PViewOptions.h’) if it's a post-processing view-dependent option;
2. in ‘Common/DefaultOptions.h’, give a name (for the parser to be able to access it), a reference to a handling routine (i.e. opt_XXX) and a default value for this option;
3. create the handling routine opt_XXX in ‘Common/Options.cpp’ (and add the prototype in ‘Common/Options.h’);
4. optional: create the associated widget in ‘Fltk/GUI.cpp’;
5. optional: if no special callback is to be associated with the widget, add the handling routine opt_XXX to the OK callback for the corresponding option panel (in ‘Fltk/Callbacks.cpp’).