Changed scripts to not infer field purposes from CSV values

Note there's a bit of subtlety here, field _types_ are still infered,
but the intention of the fields, i.e. if the field contains data vs
row name/other properties, must be unambiguous in the scripts.

There is still a _tiny_ bit of inference. For most scripts only one
of --by or --fields is strictly needed, since this makes the purpose of
the other fields unambiguous.

The reason for this change is so the scripts are a bit more reliable,
but also because this simplifies the data parsing/inference a bit.

Oh, and this also changes field inference to use the csv.DictReader's
fieldnames field instead of only inspecting the returned dicts. This
should also save a bit of O(n) overhead when parsing CSV files.

scripts/code.py

@@ -315,10 +315,7 @@ def collect(obj_paths, *,
     return results
 
 
-def fold(Result, results, *,
-        by=None,
-        defines=[],
-        **_):
+def fold(Result, results, by=None, defines=[]):
     if by is None:
         by = Result._by