Add support for multiple rings in offset_region

[mcontim]

FIX: At the moment if a country in GeoRegion has more than one Ring, the function offset_region will give an error.

[mcontim]

• _offset_polyarea now consider only the outer ring for the polygon offsetting.
• Considering inner rings would lead to further complications:
  • The offsetting of inner ring should be in opposite direction wrt te outer one (think about donut enlargement where the outer ring gets bigger while the inner smaller).
  • Split of inner polygon into more than one polygon can lead to unwanted overlaps.
• This degree of accuracy is not useful for the purpose of building a cell layout as in our case