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