new class: basin

[kaiiam]

@pbuttigieg hierarchically underneath depression a basin is required. This definition may serve for a landform basin:

Any enclosed or semi-enclosed depression within a landform.

But a more general basin definition (not as subset of physiographic feature) would be:

Any round shaped material entity serving as the site in which to contain some other material entities.

[pbuttigieg]

@pbuttigieg hierarchically underneath depression a basin is required. This definition may serve for a landform basin:

Any enclosed or semi-enclosed depression within a landform.

We should create geographic basin for now. The more general class may be more appropriately modelled as a PATO shape quality to prevent competing axes of classification that don't do much semantic work in ENVO's current form.

[kaiiam]

Hey @pbuttigieg I added geographic basin

Any enclosed or semi-enclosed depression within a landform.

To today's PR.

[pbuttigieg]

The definition will need more work later. Add an editor's note stating that the meaning of semi-enclosed must be investigated (aren't all depression's at least semi-enclosed? At what degree of inclination is a slope an enclosure?).

Minor rephrasing:

A depression which is part of a planetary crust, is of geographic scale, and is partially or completely enclosed. 

Please link with appropriate axioms to planetary crust.
I'm not sure if located in or part of is more appropriate here: topology is somewhat slippery in that regard.
Invoking something like 'geographic scale' is not something I'm a huge fan of, but it is used by physical geographers.

[kaiiam]

@pbuttigieg geographic basin definition added to #475

[kaiiam]

@pbuttigieg issue addressed in #483