DFP MultiFileSource optionally poll for file updates

[dagardner-nv]

Description

• Adds two new constructor args to MultiFileSource: watch and watch_interval, when watch is True the source will poll the input file globs every watch_interval seconds for new files.
• These are exposed as --watch_inputs and --watch_interval on the command line
• Misc updates to fix linting warnings

I spent some time looking into what the impacts of this change would be on the rest of the pipeline:

• Files shouldn't appear in the source directory until they're fully populated, otherwise the pipeline will ingest a partially populated file.
• DFPFileBatcherStage: Assuming that the watch_interval is smaller than the period argument to DFPFileBatcherStage, and assuming that new files are actually new (not historical files recently fetched) then this will cause all most new files to likely be batched together, unless they straddle the period boundary. This should be OK and is likely the desired outcome.
• DFPRollingWindowStage: This should be OK, the stage appends incoming data to the existing history for the user. However, there is a potential issue if new files are older than existing files already ingested. This could potentially happen if the files appearing in the directory could be populated from an outside source not in creation order.

fixes #975

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mdemoret-nv]

Just the one fix to counting incremental time and we should be good.

[mdemoret-nv]

Just one question.

[mdemoret-nv]

/merge