CHANGE: default of 'continuer' to 'dynamic quality'

The previous default was for the 'continuer' to be set to label
unisons that arise from stationary lower voices with '_'. Now the
default is 'dynamic quality' which sets the label to 'P1' or '1'
depending on whether 'interval quality' is set to True or False
respectively. The user can still override it with a custom string.
Some tests were modified to accomodate this change.

Related to #309.
Tests: Erased redundant test in test_ngram.py.
Tests: renamed test_settings_0 to test_settings_9.
Fixed: continuer now returns to 'dynamic quality' when it should.

When the continuer was left to it's default of 'dynamic quality',
it would change to 'P1' or '1' after running an experiment. Now
that change still happens for experiments, but after each experiment
the continuer now gets reset to 'dynamic quality' if that's what
it's setting was before the experiment.

Related to #309.
Tests: unit tests for continuer setting functionality in test_workflow.py.

One test added and two tests modified. They verifiy that the user can pass
a custom string for the continuer setting and that the continuer setting
returns to 'dynamic quality' after running an experiment if that's what
it was set to before the experiment.

Related to #309.
Tests: unit and int. test fixes for continuer in test_workflow_integration.py

Verifies correct functionality of continuer when changed to a custom
string and also when left as 'dynamic quality' both with 'interval
quality' set to True and False.

Closes #309