Priors for private germline rate in callMutationBurden

[lima1]

We currently use flat priors for private variants. This can result in artificially high private germline rates in hyper-mutated samples or low rates in silent genomes, especially in high purity samples. (Thus resulting in under- and overestimating somatic rates, respectively.)

• [ ] Provide function to estimate rate based on callMutationBurden output from many samples

• [ ] Adjust probabilities, probably only in callMutationBurden, not in fitting.