Check for note markers in the whole sheet

[matt-dray]

See https://github.com/best-practice-and-impact/gptables/issues/197.

I had interpreted the accessibility checklist (point 3) to mean:

column_heading	var_1	Notes
Inside shop	x
Street	x
Outside shop	x
In public entertainment	x	[note 1] applies to cell XX

It's possible in {a11ytables} to add a note anywhere, however; there's no proper 'note placing system'.

There is some logic that warns the user if notes are expected but not found (based on the content of the notes sheet), but it won't fire if the note is placed in a 'data cell' (it only checks for '[note x]' in the headers, or for a specific 'Notes' column).

I may need to update the note-existence check for user convenience.

(I personally don't want note markers in my data, but a11ytables can get this note-checking update at relatively little cost.)

[matt-dray]

updated the title to 'whole sheet' from 'whole table', reflecting the possibility of notes in pre-table meta content as well.