added r-spline design matrix to specfp & bug fixes

- added R spline design matrix to specfp

- Renamed parameters.c to parameters.cpp so that it compiles with g++
rather than gcc (states.h requires C++ classes)

- renamed rVec to rvec (to match EPP implementation)
added HIV stage-specific sub fertility

Note: hard-coded such that on ART < 1 year has the same fertility as
HIV+ untreated women, on ART > 1 year has constant sub fertility.

Also added update.specfp() function.
added S3 classes and generics

enables EPP and Spectrum versions to be fit with the same likelihood