removed dependency on PEcAn.priors package

I just added code for the pr.dens function; this should be less trouble than making the BETYdb app depend on an external R package. If this throws errors, please look in the files here: https://github.com/PecanProject/pecan/tree/master/modules/priors/R for missing functions. this should go into pull request PecanProject/bety#74