Changed behaviour of DefineConstant & co for better symmetry between GetDP/Gmsh

and Python onelab clients.

WARNING WARNING WARNING: This is a major change -- all our onelab-enabled .geo
and .pro files will need to be (slightly...) modified.

What's new:

1. The name of a onelab variable (in the onelab database) is no more constructed
   from the name of the corresponding GetDP/Gmsh variable. One now needs to
   specify the onelab name explicitely, using the "Name" attribute. The "Name"
   is the actual name of the parameter in the onelab database, i.e., it also
   includes the path.
   
   This makes the "Path" attribute obsolete (it has no effect anymore). The
   "Legend" attribute can still be used (and it can be useful in edge cases,
   e.g. when you want a "/" in the name of a onelab paramater), but in most
   cases it's not necessary.

2. When a DefineConstant[] & co is used and no Name is given
   (e.g. DefineConstant[a=2]), no onelab parameter is created. This allows to
   provide default values to internal parameters without polluting the database.

Why did we change?

1. The new syntax matches what we do in Python, where specifying a name is
   mandatory (there's no way around this in Python, as onelab cannot guess the
   name of a Python variable to which a onelab parameter value will be assigned).
   
   The change will prevent common mistakes where two parameters with the same
   label actually correspond to 2 different onelab parameters, due to a change
   in local getdp/gmsh variable name (which would change the onelab name
   automatically)

2. The new syntax allows to nicely decouple onelab parameters from internal
   variables with default values, that we don't want in the onelab database.