Fix type in MultiProductionDataset

[Andrew42]

Please make sure that the following items are taken care of if needed:

• [ ] Has the database format changed? (renamed or new columns, tables) Or did any of the project layout change? (files required to run the workflows)
  • Please increase the version number VERSION after the imports in lobster.util. This will ensure that Lobster does not try to load old projects with an incompatible version.
• [ ] Did the required Work Queue version change?
  • Please update the three version numbers in doc/install.rst. Adjust the tarball name in the install_dependecies.sh script, too.
• [ ] Are all additional dependencies in setup.py?
• [ ] Mark any issues as closed either in commits or in this pull request.

[klannon]

Well, that's embarrassing. Sorry!