Implemented conductivity for CO2 from Scalabrin.  All CO2 tests pass now.

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Helmholtz derivative testing gives almost 100% coverage - missing HS offset term

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
main.cxx update

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/coolprop/coolprop