Version 0.15.2

A number of bug fixes have been made in this version. One of the most
significant issues is to modify the likelihood() method of the AmpToolsInterface
class to fetch the likelihood using the same mechanism as is used by MINUIT.
This guarantees that the likelihood includes extra contributions beyond
those provided by the LikelihoodCalculator (e.g., Gaussian bounds or
user defined likelihoood contributions). A few other memory mangement issues
were fixed.