Add check_spatial_series_unit for SpatialSeries unit validation

[h-mayorquin]

SpatialSeries has a free-form .unit field that defaults to "meters" in PyNWB but accepts any string. The NWB spec says it "should be SI unit" but nothing enforces this, so files in practice contain values like "m", "cm", "unknown", or empty strings. This check flags unrecognized unit strings to push the ecosystem toward consistent values.

The valid unit allowlist uses full SI words in plural form, matching the PyNWB convention: "meters", "centimeters", "millimeters", "micrometers", "degrees", "radians", and "pixels". I chose full words over abbreviations to be consistent with what PyNWB produces as defaults and what the existing check_compass_direction_unit already expects. SI length prefixes are included following the same principle as check_subject_weight, which accepts the full range of SI mass prefixes (kg, g, mg, etc.).

The check skips SpatialSeries objects inside a CompassDirection container since those are already covered by check_compass_direction_unit.

Related to #208 (general SI unit validation). This PR addresses one specific type rather than the broader question of enforcing SI conventions across all TimeSeries unit fields.

[bendichter]

This is tricky since sometimes the units are really unknown or arbitrary, but I suppose that's fine if it's just a violation and not a critical.

[h-mayorquin]

This is tricky since sometimes the units are really unknown or arbitrary, but I suppose that's fine if it's just a violation and not a critical.

Maybe we can add a escape hatch of allowing them to intentionally write "n.a." in case that is the case, what do you think?

[h-mayorquin]

Related:
NeurodataWithoutBorders/nwb-schema#672
And
NeurodataWithoutBorders/nwb-schema#675

[bendichter]

yeah I like the idea of allowing n.a.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.13%. Comparing base (061ee1e) to head (0860fde).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #685      +/-   ##
==========================================
+ Coverage   79.92%   83.13%   +3.21%     
==========================================
  Files          48       48              
  Lines        1878     1886       +8     
==========================================
+ Hits         1501     1568      +67     
+ Misses        377      318      -59     

Files with missing lines	Coverage Δ
src/nwbinspector/checks/__init__.py	100.00% <ø> (ø)
src/nwbinspector/checks/_behavior.py	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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