updated n_polyA_umi.py and tests

[as2110]

To test this function use the scripts in the dev in this order for simple testing of the functions:

1. find_3prime_adapter_test.py - use A001.sub.1k.fa as infile and n=1 for first read.
2. find_longest_polyA_test.py - use A001.sub.1k.fa as infile and n=1 for first read.
3. extract_and_correct_umi_test.py - take the segment output in terminal from the find_longest_polyA_test.py (the UMI window)
4. full_polya_umi_extraction_test.py - use A001.sub.1k.fa as infile and n=1 for first read, outputs written to files.

Together these are incorporated in n_polya_umi.py script which the pipeline_counts has been updated to use.