Add check for Units table without spike_times

[h-mayorquin]

The Units table in NWB exists to describe sorted neural units, and spike times are its core data. A Units table without a spike_times column is almost certainly an error or a misuse of the neurodata type (for example, storing cluster labels or quality metrics without the underlying spike data that gives them meaning).

This PR adds check_units_table_has_spikes at CRITICAL importance to flag this case.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.06%. Comparing base (60bfe7f) to head (8b769a7).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #691      +/-   ##
==========================================
+ Coverage   79.87%   83.06%   +3.19%     
==========================================
  Files          48       48              
  Lines        1873     1878       +5     
==========================================
+ Hits         1496     1560      +64     
+ Misses        377      318      -59     

Files with missing lines	Coverage Δ
src/nwbinspector/checks/__init__.py	100.00% <ø> (ø)
src/nwbinspector/checks/_ecephys.py	94.11% <100.00%> (+0.17%)	⬆️

... and 5 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.