ATAC-seq support

The main challenge with using this pipeline for ATAC-seq data is the use of STAR for mapping within the WASP subworkflow. We could create two new mapping rules within the WASP subworkflow that use `bowtie2` (or `BWA`), instead. And a new config option could be created to switch between the different aligners that we offer.

As for the variant calling pipeline, we could update it to use [VarCA](https://github.com/aryarm/varCA) and have it use different options when executing `BWA`. The only problem with using [VarCA](https://github.com/aryarm/varCA) at the moment is that it doesn't output GQ tags, but issue #60 would resolve that.

Start by reviewing the WASP subworkflow's mapping rules and the variant calling pipeline described in the issue. Check the proposed bowtie2/BWA aligner choices, the new configuration option, VarCA's GQ limitation, and issue #60; done means ATAC-seq processing supports the required mapping and variant-calling paths.

Written by the indexing model from the issue text.