Stratigraphic column overburden & underburden

[andy-beer]

RESQML v2.0.1 specifies that a stratigraphic column ends with the topmost and basal stratigraphic units, without the horizons above and below those units. As it is common to want to end the column with the horizons, the usage guide for v2.2 includes dummy overburden and 'underburden' stratigraphic unit interpretations (and related features) which can be used as the end units in the column. These dummy unit organisational objects have fixed UUIDs.

The issue is to add the option of using these dummy objects in resqpy when working with a stratigraphic column.