Add _desc options to loadmesh

* Add the missing desc option to pass a custom Gmsh pointer
* Default behaviour doesn't change.
add vector() function
Improve parsing option of GiNaC expression

Now, symbols can be specified through the options (still as a string).
The string format is "expression,symbol1,symbol2,..."

example:
auto foo = expr( "cost(x*y),x,y" );

Also numerical expression are supported.

example:
ex foo = expr( "cos(4*Pi)" );

See qs_laplacian.