Add module to generate BSgenome --> enables bowtie as default off-target search

[m-jahn]

• right now biostrings is used to search off-targets as it has no dependencies
• bowtie requires a BSgenome object
• can be made with BSgenome R package

[m-jahn]

• after experimenting with BSgenome package, it turned out that making a BSgenome object on the fly is not trivial in snakemake
• problems with path resolution, hard to debug
• will postpone until a fix or simple solution is available