Add optional parameters for virus-specific and serum-specific drift rates.
Add optional parameters for virus-specific and serum-specific drift rates.

These are virusDrift and serumDrift.  If set, they override locationDrift.
JifengJiang/beast-mcmc,JifengJiang/beast-mcmc,JifengJiang/beast-mcmc,JifengJiang/beast-mcmc,JifengJiang/beast-mcmc