Improve default region mapping

Search for the region mapping, originaly from Gleb, corrected by Andreas Spiegler.
This region mapping has indices for 76 regions. The two extra regions represent the corpus callosum or the medial wall and where added to the connectivity matrix.

The proposed short-term solution is to use this corrected version with 76 regions, with the connectivity matrix being set to 0 for the 2 regions corresponding to the corpus callosum

The long-term solution involves more work including:
+ having a new attribute to identify the vertices belonging to the medial wall.
+ handling the geodesic distance, so the vertices of the medial wall are far and thus the local connectivity kernel falls to 0.
+ masking the activity of those vertices.
+ choosing between asking users to add the 'fake' regions into their connectivity so len(unique(region_mapping)) == number of nodes in the connectivity; or, handling it at configuration time.