Docs: fixed rtype description for interval n-gram experiments in workflow.py

The documentation was incomplete because when an interval n-grams experiement
is run with "count frequency" set to False, the return type is a list of lists
of series but this was missing in the documentation.
Fix: double-or-higher octaves in simple interval mode in interval.py

Double octaves (i.e. 15ths) and triples etc. were previously counted as
unisons in simple octave mode but now they get simplified to octaves using
music21's "semiSimple" feature.

Closes #312
Tests: Add regression tests for 275929e0d548d9b73ad8ee28434ebaed75e68b51
Tests: adjusted a test to account for recent correction of simple intervals.
Merge branch 'experimental_simple_intervals' into master
Merge branch 'master' into new_workflow_order
Merge branch 'new_workflow_order' into continuous_lilypond