Defining regions without specifying center or shape parameters

In #430, I added validation to the shape parameters requiring `radius`, `width`, and `height` parameters to be strictly positive (> 0). That makes complete sense, but it broke @adonath 's workflow where he had been using `np.nan`s as placeholders in some cases (https://github.com/astropy/regions/pull/430#issuecomment-1076349753).

This topic came up before (by me 6 years ago) in https://github.com/astropy/regions/issues/56, but it was never implemented. The workaround of course is to define the region with valid (non-nan, non-zero) dummy values, which can be changed later (the center and shape parameters are mutable). I think this needs discussion with other devs.

@keflavich, @astrofrog, @dhomeier -- any thoughts or ideas?

Start by reading issue #430 and the earlier discussion in issue #56, including the linked pull-request comment, to understand why strict positive shape validation conflicts with NaN placeholders. The issue does not identify files, tests, or a settled behavior, so the desired definition semantics and completion criteria still need discussion with maintainers.

Written by the indexing model from the issue text.