BUG: Handle iterable of arrays in convert
DatetimeConverter.convert can take an array or iterable of arrays.
Fixed the converter to detect which case we're in and then re-use
the existing logic.

Posting here for feedback from @tacaswell. Specifically,

1. Presumably our other two converters `TimeConverter` and `PeriodConverter` could suffer from the same problem?
2. `if is_list_like(values) and is_list_like(values[0]):` feels hacky... Thoughts?

xref https://github.com/matplotlib/matplotlib/issues/8459