Bug fix for Fast5_to_seq_summary for directories with 1 fast5 file

[godotgildor]

Currently, the code looks at the enumerate index i to see if a fast5 file was found in the given directory. However, if a directory does not have any files in it, then the enumeration loop won't get started and the i variable will actually be unbound, so the code as written would raise an UnboundLocalError. If there is a single fast5 file in the directory, then i will be valid, but it will be ==0, causing the current code to error saying that no fast5 files were found.

I've updated the logic to use a simple flag to indicate when we have actually inserted a fast5 file into the Queue during this loop.