IO format selection

[wpbonelli]

We anticipate supporting different formats for input files, including at the moment ASCII and netCDF.

We need a way to select between these at write time. One way might be a format parameter in the load and write methods. This follows astropy. We could also follow their registry pattern more broadly. This seems like an achievable first step in the near term.

We also need finer control over how variables are written individually and/or for specific components. imod-python solves this at the component scope with a dataclass passed into internal write routines. I wonder how we might extend this to individual variables. If we implement load/write with a descriptor like astropy, I think we could have arbitrary methods/properties on it. E.g. they have help().

For now, at least to get the quickstart running, we can write everything to ASCII with internal arrays/lists etc.

[wpbonelli]

@deltamarnix raised the question of whether format choices "trickle down" from parent to child components