Polya trimming correction

Closes issue #98 (closed) Modified the samples.tsv to include poly-a for the 3p and 5p to match the cutadapt options -a -g -A -G. Empty variables are substituted by 'XXXXXXXXXX' so that they are not trimmed by cutadapt.

tests/test_scripts_labkey_to_snakemake_api/expected_output.md5

 b163e7b06bd9e0a71f2fd1fc4935fea9  config.yaml
-cb58e046242c2702038e6e21dbd0bdb4  samples.tsv
+071f0e942321df5e38c8b2d458f7be06  samples.tsv