Various changes focused on improving accuracy of effect size estimation.

  * Bugs fixed in tpbias and gcbias application.
  * Gamma priors are used for sigma parameters to more vigorously
    resist variances shrinking towards zero.
  * Model hyperparameters are exposed on the command line.
  * Some simulations were used to guide the choice of reasonable
    values for the prior hyperparameters.
  * Misc tweaks.