dppPowerExp(...)tag=value specifying the
    parameters. See Details.lambdaas a positive numericalphaas a positive numericnuas a positive numeric
    (artificially required to be less than 20 in the code for numerical
    stability)das a positive integer"detpointprocfamily".dppBessel,
  dppCauchy,
  dppGauss,
  dppMatern