ADD: Some bugfixes to get the unit test to work.

[rcjackson]

Code Bugs Found

1. radclss_core.py:476 — nexrad_columns.drop() when nexrad=False
   nexrad_columns is None when nexrad=False, but .drop() is called unconditionally.
   nexrad_columns = nexrad_columns.drop(["time_offset", "base_time"]) # crashes if nexrad=False
   Fix: wrap in if nexrad_columns is not None:.

2. radclss_core.py:409,411 — .resample() not followed by aggregation
   ds_concat[k].resample(time=time_coords) returns a DatasetResample, not a Dataset. Then .reindex() is called on it, which crashes with AttributeError. This code path is hit by test_radclss_serial_with_5min_time.

3. radclss_core.py — volumes["sonde"] KeyError when sonde not provided
   volumes["sonde"] raises KeyError if sonde files aren't included (e.g. test_radclss_parallel_with_nexrad).
   Fix: volumes.get("sonde").

4. Will no longer crash on no sonde data.

5. No need for support for time averaging at this point after coordination with the mentors.