odelia: ODE solver using Rune-Kutta 4-5 adpative step, C++ header files

The odelia package for R is an ODE solver in C++ header files, using RK4-5. There's potenital to link to R via Rcpp. Code was first developed by Rich FitzJohn as part of the plant package. I'm spinning that code out into a package, as I want to use it elsewhere.