Rename `project_dirs`

[tcompa]

Should these be called base_zarr_dirs? cc @jluethi @lorenzocerrone

[jluethi]

Yes, we should find a name that better fits the scope of the current project_dirs. base_zarr_dirs sounds too technical for me. Here are a few alternatives (brainstorming, I haven't found the one yet that seems very natural to me):

Centered around datasets:

• dataset_homes
• dataset_zones
• dataset_roots
• dataset_locations

Centered around zarr_dirs:

• zarr_dir_zones
• zarr_dir_roots
• zarr_dir_bases