Add `ncdfgeom` class attribute to list created by functions.

[dblodgett-usgs]

In order to move on #65 it will be helpful to have a method for the ncdfgeom list as an object.

This object may be temporary in the package as there is a chance that ncdfgeom can just use stars objects directly rather than working with the old list format. For now, just adding a class to the list will be backward compatible and allow things to move forward.