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