Raise RuntimeError when WorkflowManager.load() Has Invalid Instruction

Now load() behaves like run(), output(), and export() in reporting
unrecognized instructions. This also prevents the self._loaded property
from being set to True when nothing was loaded but an invalid instruction
was provided.
Add "count frequency" Setting to WorkflowManager

Building toward issue #268.

Set "count frequency" to False to avoid the aggregation and frequency-counting
steps that are the default behaviour.
The Experiments Pay Attention to "count frequency" Setting

Working toward issue #268.

The experiments will avoid calling _run_freq_agg() as required, and there
are tests for it.
Bugfix WIP: Correct WorkMan Int N-Grams Experiments

- update the docstrings to say they no longer run frequency/aggregator
- append per-piece results (in a Series) to a list that is returned
Update WorkflowManager._intervs() Documentation

In the spirit of the previous commit, this method also no longer runs the
frequency and aggregation experiments.
Fixed WorkflowManager._interval_ngrams() for Previous Commit

And its tests... and the integration test...