Expose makeValid

[tdunning]

I see that there is a function LibGEOS.GEOSMakeValid in the generated code.

Unfortunately, when I call it like LibGEOS.GEOSMakeValid(Ref(p)) where p is a self-intersecting polygon, I get back a null pointer.

This would be a very nice function to have working, but I don't understand how call it (else I would have brought a pull request).

Is there any way to get this to work?