Advanced gapped kmer, IC-based motif centering

[AvantiShri]

Implementation for advanced gapped kmer embeddings, IC-based motif centering in the "expand-trim-expand" postprocessing steps, some other minor fixes/features (including ability to supply custom per-pos contrib score tracks).

To use advanced gapped kmer embeddings, supply seqlet_embedding.advanced_gapped_kmer.AdvancedGappedKmerEmbedderFactory() as the embedder_factory argument in tfmodisco_workflow.seqlets_to_patterns.TfModiscoSeqletsToPatternsFactory (which is in turn supplied as the seqlets_to_patterns_factory argument of tfmodisco_workflow.workflow.TfModiscoWorkflow)