Add functions for regional analysis of the mesh

[p-j-smith]

Changes made:

• Added a case.fields.region attribute - stores the region id of each cell in the mesh
• The region ID is used in regional analyses -
  • Added openep.mesh.mesh_routines.low_field_area_per_region for calculating e.g the low voltage area per region
  • Added openep.mesh.mesh_routines.mean_field_per_region for calculating e.g. the mean voltage per region
• Added test that region ID is imported and exported correctly
• Added tests for regional analysis functions