Fixed buglet thrown up by R CMD check
Print/show method for ode_system
Simple show method for steppers
Rename type -> algorithm

Should be a bit clearer and stops all sorts of confusion
in the actual c++ code about what a "type" is.
Don't hardcode package name into Makefile
Make size_t/int changes explicit
Check pointer validity before dereferencing.

Should protect from all crashes, minimal overhead.
Primitive support for serialisation of ode_system
Basic serialisation for stepper types