New feature: MEME-AME transcription factor motif enrichment with customizable background sequences

[fwzhao]

Features:

Background: default - control sequences (e.g. cell type background) matched to lengths of test sequences OR dinucleotide shuffled background of test sequences. May be better to use shuffled as default

Integration into results aggregation & visualizations

Known bugs:

Proper tracking: e.g. if MEME-AME gets cancelled and empty results files are generated, then snakemake doesn't restart these jobs