Region mapping refactoring

Created by: BalanceKey

Refactoring the region mapping definition, by using the method datatypes.RegionMapping.full_region_mapping(surf, conn), which uses the surface of the cortex and the connectivity matrix, thus mapping both the cortical and the subcortical regions. The goal of this is to make it easier in the future to add modifications to the way the region mapping is realized, by just changing this one method.

Actually, the method datatypes.RegionMapping.from_file is also used in other cases where the region mapping is retrieved from a .txt file. These are at the moment the two ways to define a region mapping.