Simple-file-poller

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- added the `output_num_files` boolean property to the `Poller` class, which extends the logging output by
outputting a `x/y` string as well (x=current file index, y=total number of files in a batch)

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- the `Poller.logging` property no longer returns the `check_file` method

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- fixed high CPU usage (ie constant polling) when using unlimited files to list (`max_files=0`)
- `poll_wait` and `watchdog_check_interval` are now float instead of int to allow for sub-second poll waits
- internal delays when poller is busy have been dropped from 1s to 0.1s

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- `keyboard_interrupt` method is now public

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- fixed race condition between watchdog reacting to new files and watchdog checking for files at specific intervals
- logging now distinguishes between DEBUG/INFO/ERROR levels
- keyboard interrupts via CTRL+C are now handled correctly
- in watchdog mode, an initial scan of the input directory is now performed, in case files were already present

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- added `params` object to allow attaching of arbitrary parameters to be used by the `check_file`
and `process_file` methods; this avoids accidentally overriding Poller attributes that were
introduced in later versions.