Add "skip denovo" option for mutational signatures

[sjspielman]

The mutational signatures module cobbles together several distinct analyses, many of which are not part of the manuscript but take significant of memory and time. To facilitate running the most relevant parts of the the mutational signatures module, we might consider adding an option to the script to skip de novo fitting.

[sjspielman]

Welp, looks like this is actually already there as an option CNS_FIT_ONLY and I had magically glossed over it!