Refactor: remove unnecessary unicode literals in workflow-related files

Maybe I'm thinking we should use Python 3, who knows?
Correct workflow-related file headers
Refactor: WorkflowManager._remove_extra_pairs() for VIS 2

The method now works on a DataFrame rather than a dictionary.

Related to #325
Two minor fixups in experimenter.frequency

Found it while I was trying to figure out something else.
Refactor: WorkflowManager._run_freq_agg() for VIS 2

I modified the method to work with the admittedly cumbersome VIS-2 versions
of the relevant experimenters. The method now also requires an argument,
telling it which index to give to the FrequencyExperimenter for counting.

Related to #325
Refactor WorkflowManager._intervs() for VIS 2

I rewrote the tests too, and they're better tests than before. I think.

Related to #325