Suggest "presence" or "coverage" instead of "geometric_opacity"

I was wondering if anyone would be open to using an alternative name for `geometric_opacity`.

In OpenPBR the intention of this signal is to create cutouts of the geometry, rather than to change how much light is transmitted through the surface. In my experience opacity is usually used to describe the degree of transmittance.

Interestingly, the definition of the parameter mentions presence:

> where α = 𝚐𝚎𝚘𝚖𝚎𝚝𝚛𝚢_𝚘𝚙𝚊𝚌𝚒𝚝𝚢 is the presence weight of the entire surface

In my opinion choosing `presence` would improve the readability of the parameter. `coverage` is another alternative if `presence` isn't appropriate for some reason.

Review issue #138 and the quoted OpenPBR definition of geometric_opacity, including the proposed presence and coverage names. Done requires a resolved naming decision and corresponding specification updates, but the issue provides no file or test entry point and has had no recent activity.

Written by the indexing model from the issue text.