very small: changed file path to library in functions.R
moved fertility calcuation to subfunction, added function to calculate output prevalences
corrected missing /2 in ll.hhsurv
add likelihood function to C++ code

This implements likelihood function for r-spline model with national adult and
ANC prevalence for South Africa. Prior and prior sampling following Dan Hogan's
r-spline code.

- C++/likelihood.cpp: functions for likelihood, prior, and sample_prior.
- C++/test-prior.cpp: tests sample_prior, and prior and likelihood functions.

- C++/test.cpp: added to this to run the model and calculate likelihoood
                for a set of input parameters.
- R/test.R: updated to reproduce same examples as test.cpp