Fixed mixed_MCMC

Found the mistake which was that pi was not being updated properly.

Also added code to the function so that contamination can be set to FALSE, and the MCMC can just be run by updating pi and u.
Updated Worklog

Includes information on the different tests I ran to determine if mixed_MCMC was running properly.
Changed Functions so they work with rho = 0

Changed population_compare so that it is compatible with rho = 0. Also changed the make_mixture function so that it make populations with 0 contaminated samples.  Made a few other minor changes to make_mixture.