Estimate priors for each hypothesis

`hypothesis.priors(nsnps, pn, pa, pc)`

- nsnps
number of SNPs

- pn
prior probability that none of the SNPs/variants in the region are associated with the query trait

- pa
prior probability that a non-query variant is causally associated with the query trait

- pc
prior probability that the query variant is causally associated with the query trait

hypotheses priors