Only use products that exist on disk when figuring inputs

[jtniehof]

This PR changes _getRequiredProducts to only consider files that are recorded in the database as existing on disk, and thus won't try to use nonexistent files as inputs to processing. Closes #47.

I think I had speculated somewhere that the runMe checks of the command line build would catch trying to build with a nonexistent file. Turns out that's not the case, and I haven't chased that down.

89 captures additional possible behavior changes that were incidentally mentioned in #47.

PR Checklist

• [X] Pull request has descriptive title
• [X] Pull request gives overview of changes
• [X] New code has inline comments where necessary (N/A)
• [X] Any new modules, functions or classes have docstrings consistent with dbprocessing style (N/A)
• [X] Major new functionality has appropriate Sphinx documentation
• [X] Added an entry to release notes 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 (use Closes # if this PR closes the issue, or some other reference, such as See # if it is related in some other way)

[jtniehof]

I've reprocessed several days of PSP data which were affected by this bug (we had a problem where our L0 files were not split on the appropriate day boundaries, so obsolete versions of the L0 were across the wrong day.) All worked fine.