Multiple materializations in one statement

[lucananni93]

Creation of a gmql.materialize(datasets) which enables the execution of multiple materializations, both in local and remote mode.

datasets can be:

• a list of GMQLDataset: in this case, the result is loaded directly from memory and a list of GDataframe is returned in the same order as in the input
• a dictionary of the type { output_path: GMQLDataset }: in this case, the result is saved on the disk at the locations specified by the dictionary keys and a dictionary of the type { output_path: GDataframe }

The execution of the query must be unified for all the materializations.