[toakinrele]

Adds support for an explicit weights_file configuration option so precomputed weights can be supplied independently of gpkg_file.

Changes:

• weights_file takes precedence when provided.
• Existing gpkg_file behavior remains as the fallback.
• gpkg_file is still used for hydrofabric-dependent operations such as plotting.
• Consolidated plotting validation logic.
• Added a test for the weights_file override functionality.

Testing:

• test_weights_file_override passes locally.
• test_plotting passes locally.
• CSV, Parquet, and NetCDF output tests pass individually.

Closes #45.

[JordanLaserGit]

Looks good @toakinrele! Thanks for this work. We can ignore both failing tests in the pr as one has been documented here #101 and the other here #103.