First pass at cleaning up lists in the .geo parser: 

- we now keep track of the variable type ("list of floats" or "single float")
- the brackets [] after the variable id are now optional (aa={1,2} == aa[]={1,2}) when creating a new list
- the old syntax is kept to enable forcing the type to "list of floats" when creating a single-entry list
- on single floats, += performs the usual arithmetic operation; on lists, += appends to the list (idem for -=)

WARNING! this is a possibly dangerous commit: please test your .geo files and your scripts and signal any weird behaviour