Fitting stationary background objects

[tomasstolker]

This PR adds a few things:

• Fitting a stationary background object by setting ndim=2
• Some rewriting of the Gaia query to find the target properties. It queries the target directly instead of a region that finds multiple sources.
• Support for latest astroquery and Python 3.12

Add some point we may want to implement a different way for the number of parameters + priors, such that individual parameters can be added and their priors adjusted.

[wbalmer]

This is great, thank you especially for helping with the latest astroquery and python update. This will implicitly set an astroquery version requirement (astroquery>=0.4.9, which thankfully works with python>=3.9 <3.13), so I'll made that explicit in a future commit. I'll also update the examples with the stationary ndim=2 fitting.

I agree about making the number of parameters/priors more flexible, I can open an issue to track that.