Some bug fixes

Added

• Added new option to force overwrite the version considered by the legacy resolver for testing purposes.
  At the moment this is only supported for Datasets and not Sessions.

Fixed

• Converting and saving old legacy files into the new 0.18 format now works correctly.
• The Ipython representation of the header now ignores fields that throw error.
  This fixes an issue that the representation could not be displayed when no timezone was specified.