Fix GML import - try to keep column format double, if the parser detected it as double

``GML`` is quite a convinient format when creating graphs from scripts. I need to pass several numerical attributes together with the nodes and edges, and this patch removes the pain of manually clicking ``copy column`` and changing it's type to ``Double`` for each of the columns.

Not much error checking is done, but in script generated graphs typo errors do not happen very often;)