Warn if table is not rectangular

[DavidGinzberg]

from source:

...
      cols = row.findAll(['td','th'])
      print(f"Found {len(cols)} columns.")
      #TODO: Detect and discard empty tables (those that contain 1 row, 1 column, consisting of an empty string...I think?)
      #TODO: Add a warning for non-rectangular tables.
...

Tables with varying numbers of columns per row may be a sign that there are layout tables, errors, or multicolumn headers (not currently supported). Produce a warning when these are seen (and allow it to be disabled or cause script to abort, both based on command flags)