Introduce Point_set class

[ole]

It may be desirable to have a class Point_set which contains collections of points with possible attribute values.

This would be very useful for set_quantity but also for allowing points to have methods for georeferencing and mappings between latitude/longitudes and UTM as in create_mesh.py in the Karratha study.

Point_set would point to geo_ref object and know whether values are relative to that or absolute. Points can also be geographic (lat/lon)