Support wrfinput and nested WPS grid spacing

[mefrediani]

Summary

• accept wrfinput mass-grid coordinates via XLAT/XLONG in addition to WPS XLAT_M/XLONG_M
• derive nested-domain dx/dy from scalar WPS parent-domain spacing and parent_grid_ratio
• document wrfinput support and add focused unittest coverage for nested geometry handling

Validation

• TMPDIR=/glade/derecho/scratch/frediani/tmp /glade/work/frediani/casper/anaconda3/envs/py314/bin/python -m unittest discover
• /glade/work/frediani/casper/anaconda3/envs/py314/bin/python -m py_compile fire_preprocess.py fire_preprocess/cli.py fire_preprocess/namelist.py fire_preprocess/wrf_domain.py tests/test_namelist.py tests/test_wrf_domain.py
• git diff --check

Note: full CLI import was not validated in py314 because that environment lacks rasterio; py311new has rasterio but lacks f90nml.