ENH: distinguish OpenFOAM version for user-coding (fixes #195)
The pre-processor macro 'OPENFOAM_PLUS' is defined with a numerical value equal to the currently compatible version number.
This can be used judiciously within user coding to help with minor differences between OpenFOAM versions. For example,
#ifdef OPENFOAM_PLUS
#if (OPENFOAM_PLUS >= 1612)
...
#endif
#endif
or simply
#if (OPENFOAM_PLUS >= 1612)
...
#endif