ENH: Handling open RANGE intervals for POS options: handling Inf

I was following up on recent reports of SIAv2 inconsistencies, and run into the issues that currently we don't allow `+/- Inf` for POS values, though that standard (e.g. SIA2 2.1.1 section) lists them as valid values for e.g around the poles or for all sky.

Should we allow these? It will need a bit of a hackery in the validator, but can make it work.
```
# North pole: (0, 360, 89, +Inf)]
# All sky: (-Inf +Inf -Inf +Inf)
```

(Also, with the very recent #594 it became clear that we have these position parsers both in DAL and in registry. Now the two are not exactly the same, so may not worth spending too much effort to cleanup, but still, we may consider doing it at some point.

Read the POS validators and position parsers in the DAL and registry areas, then compare their current handling of finite values with SIA2 section 2.1.1. Use the North Pole and all-sky examples in the issue as cases; done means valid open ranges containing +/-Inf are accepted consistently, with the parser differences understood or addressed.

Written by the indexing model from the issue text.