Fixed: WorkflowManager.output() acts differently when "count frequency" is False

This is for table-like outputs only: CSV, Stata, Excel, and HTML.

Previously, output() would fail if called with table-like output formats but the
"count frequency" setting were False. Now, it outputs the unaggregated results,
one file per piece, quasi-LilyPond-style.

This is required for "moment-by-moment" output in the Counterpoint Web App.
Fixed: WorkflowManager._make_histogram() detects previous experiment properly

Previously, this method was looking for "n-grams" as the previous experiment,
though it should have been looking for "interval n-grams." This is now fixed.
Copied bugfix line in VERSION_HISTORY from the 2.0.1 release
New: WorkflowManager.output() dynamically adjusts column name

Previously, the results of an aggregated and frequency-counted experiment
would always result in a table with a column named for the ColumnAggregator.
Now, it's changed to "Interval Frequency" or "Interval N-Gram Frequency,"
depending on which experiment was most recently run.