Correcting origin when it is younger than initial tree height in SABirthDeathModel

[alexeid]

It would be nice if an incorrect initial origin value could be automatically corrected (when possible, and with a warning) instead of an exception being thrown. Sometimes when a tree initialiser is being used it is not possible ahead of time to know what value of origin is large enough.

I suggest replacing the current exception with these two lines:

  originInput.get().setValue(rootHeight + 1.0);  
  System.err.println("WARNING: Initial value of origin was changed to " + origin() + " so that it is greater than initial root height (" + rootHeight + ")");

@rbouckaert is working on a solution that would allow a check on the RealParameter to make sure it is being estimated. Then if it is, the above solution can be employed, else throw exception.