org-plot: Handle explicit date/time index

org-plot: Handle explicit date/time index

* lisp/org-plot.el (org-plot-quote-tsv-field): Dump data as is when
  `timeind' is set.

* lisp/org-plot.el (org-plot/gnuplot): By-pass type checking when
  either `textind' or `timeind' is set.

The current org-plot relies on `org-table-number-regexp' and
`org-ts-regexp3' to check the index type.  However, we already have
`timeind' to indicate that the index is the datatype of date/time.

By-pass the type checking in `org-plot/gnuplot' when either `textind'
or `timeind' is set.  The whole point of the check is to determine the
type. We do not need that when we already know.

Also dump the date/time data with `org-plot-quote-tsv-field' as is,
without double quotes, when `timeind' is set.  So that we can use
`timefmt' to read what we have.  Currently we do not have a good way
to specify `timefmt' with the double quotes.

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