[fparser2] Link #if ... #elif ... #else ... #endif preprocessor directives

[reuterbal]

This is born out of #215 .

With the added support for preprocessor directives it would be an advantage if the directives for conditional inclusion could be linked in some way. A working implementation that can cover cases where the entire if...endif block is within a Fortran block can be found (commented out) in PR #239 .

However, since these can potentially span multiple blocks or be completely orthogonal to that, this does not work in a general setting. Instead some other linking mechanism (with a somewhat looser coupling in an informal rather than structural sense) might be needed.