addFile: fix utc_start_time/stop_time as dates with Unix time table

[jtniehof]

addFile used to work if the utc_start_time or utc_stop_time were passed in as a date instead of datetime--it just put to midnight of that day. It's a little bit weird, but the inspector output tends to get passed straight to addFile and I managed to write a few inspectors that did this in some circumstances. I figured if I could make that mistake it might happen again. With #31 this no longer worked, since they were treated as datetimes before hitting the database. This PR explicitly converts to datetime.

PR Checklist

• [X] Pull request has descriptive title
• [X] Pull request gives overview of changes
• [X] (N/A) New code has inline comments where necessary
• [X] (N/A) Any new modules, functions or classes have docstrings consistent with dbprocessing style
• [X] (N/A) Major new functionality has appropriate Sphinx documentation
• [X] (N/A) Added an entry to CHANGELOG if fixing a major bug or providing a major new feature
• [X] New features and bug fixes should have unit tests
• [X] Relevant issues are linked in the description (e.g. See issue # or Closes #)

[jtniehof]

See #37 for an attempt to address root cause.