add continuity optional arg to TimeSeries.__init__

[bendichter]

Motivation

develop API for NeurodataWithoutBorders/nwb-schema#425

depends on hdmf-dev/hdmf#334

How to test the behavior?

TimeSeries(name='my_name', description='my_description', data=[1,2,3], timestamps=[1,2,3], continuity='step')

TimeSeries(name='my_name', description='my_description', data=[1,2,3], timestamps=[1,2,3], continuity='result_in_error')

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using #XXX notation where XXX is the issue number?

[bendichter]

I wrote a one-off controlled vocab validator, but it might make more sense to write a general validator in hdmf and import it here. Something like:

def validate_controlled_vocab(self, kwargs, arg, allowed_terms, optional=False):
    value = kwargs[arg]
    if optional:
	allowed_terms.append(None)
    if value not in allowed_terms:
        raise ValueError(
            '{} must be one of: {}. Received value "{}" for {} named {}'.format(
            arg, controlled_vocab, value, self.__class__.__name__, kwargs['name']))

[bendichter]

or add "allowed_values" to docval