DM-44261: Add support for parsing IVOA POS string

[timj]

@gpdf here is a quick implementation for parsing IVOA POS strings.

• I copied the continueClass code from lsst.utils since I'm not sure we want to add utils as a dependency.
• I put all the code in __init__.py since the package is a bit oddly designed in terms of the code all coming from C++.
• There are no real tests that I did things properly since writing a test that the range/circle/polygon is correct is a bit tricky since if I have implemented this incorrectly I would mostly be creating a sphgeom Region in the same incorrect way in the test. Ideally I need a third party to write some tests.

cc/ @TallJimbo

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[andy-slac]

Looks good, few minor questions/comments.

[timj]

I have added Region.to_ivoa_pos() so you can now generate POS strings from a region.

[timj]

@andy-slac I changed enough that it's probably worth a quick once over again. Thanks.

[andy-slac]

Looks great, a couple of minor comments.