Experiment.subset doesn't support a single Str condition

[jeffhussmann]

The docstring for Experiment.subset describes support for providing a single condition as a string, but commit b71b1f8 added code to immediately cast the conditions argument into a list so that the code branch for handling a string can never be hit:

cytoflow/cytoflow/experiment.py

Lines 233 to 247 in c8e40ee

	conditions = list(conditions)
	if isinstance(conditions, str):
	c = conditions
	v = values
	if c not in self.conditions:
	raise util.CytoflowError("{} is not a condition".format(c))
	if v not in list(self.conditions[c]):
	raise util.CytoflowError("{} is not a value of condition {}".format(v, c))
	else:
	for c, v in zip(conditions, values):
	if c not in self.conditions:
	raise util.CytoflowError("{} is not a condition".format(c))
	if v not in list(self.conditions[c]):
	raise util.CytoflowError("{} is not a value of condition {}".format(v, c))

If you could clarify the intended behavior here (and what b71b1f8 was fixing), I would be happy to submit a pull request restoring string support.