Allow users to specify a default timezone for date/time values

[kal]

This was a feature in LevelUp. Allow users to select a default timezone to use when parsing date/time values with no timezone specifier. This allows the converter to either accurately convert date/time values to UTC or to generate properly specified date/time values with a timezone when there is no timezone information in the CSV itself.

Ideally this should be set in the metadata by the metadata editor and passed through to the worker that way so that it is possible to later see from the metadata what timezone was applied.