Incbet fix

Fixes #3761. 

The problem is that the incomplete beta function needs to calculate the beta function but was doing so semi poorly. It should just be calling the beta function since that is where we try pretty hard to calculate it well for all values.  Secondly, the value of the constant MAXGAM used in beta.c was too small.  This compromised the results for moderately sized inputs.  I also changed how we decide what order to evaluate the ratio of gammas since this fixes some failures revealed by the increased MAXGAM.