snakemake .tmp files

[seichhorn]

Just wanted to mention that I noticed that I've built up a lot (85 Gb) of .snakemake/tmp.* files in my MERlin directory. I'm assuming this happens because we don't lock the directory, so if an analysis fails there's no need to remove this prior to running the next instance. I'm not sure what would be safe to use to eliminate these directories at the start of an analysis, but I think it might be worth including something to do this.