Add documentation and interpolate example.
Correctly clone test cases and try stack in copy_members(), and free try srack in free_members().
Minor adjustments.
Correctly clone and free test suites.
Rename GObservations_optimizer.cpp to GObservations_likelihood.cpp.
Put likelihood computation in GObservation class.

The likelihood computation has been removed from GObservations::likelihood and has been put in the virtual GObservation::likelihood method. This method implements standard likelihood computations for binned and unbinned analysis. It may be overloaded to implement observation specific likelihood computations.