ProbtrackX2 interfaces, fixes #792

Together with @medihack we generated an interface for probtrackx2. Because of redundancy and conflicts with the probtrackx interface we moved common inputs to a base class (ProbTrackXBaseInputSpec). We didn't change the test file, because it contains the warning:  # AUTO-GENERATED by tools/checkspecs.py - DO NOT EDIT.

could we add the `network` option to this?