Update Geospace handler to expect numeric georegion feature IDs

N.B. Please leave merging the branch to me (Urmila) - without running the appropriate migration script immediately afterwards (which I am still working on), staging will be broken.
rojoma-json doesn't have built-in `Option` parsing for non-top-level `Options` (and here it's not top-level) because there is no preferred unambiguous encoding for it in JSON.  In _particular_ cases value-or-`null` can be used, but that cannot represent nested options, which a general codec for `Option[T:JsonCodec]` would need to do, since it asserts that it works for _all_ `T`-with-a-`JsonCodec`, which includes `Option[U:JsonCodec]`, and that would be lossy.  In this _particular_ case it's safe, because the only thing it's being used for is `Option[Int]`.  I'd actually prefer that this codec be specialized to Option[Int] for that reason.

Doubling `Some`s here is obviously being done because a `JsonCodec` returns `Some(thing)` to indicate a successful decode, and what is being successfully decoded is `Some(realThing)` -- thus `Some(Some(realThing))`.