Move simulations to MegaBEAST

[karllark]

Simulations are currently done in the BEAST code. This requires generating a new physicsmodel for each change in the BEAST priors.

The MegaBEAST work has provided code to update the physicsmodel "prior_weight" without needed to regenerate the full physicsmodel file. Moving the simulations to the MegaBEAST would speed up simulations. This update should use the megabeast settings format to specify the ensemble parameters (=BEAST priors) and update the prior_weight on a previously computed BEAST physicsmodel.