Avoiding callback by string-based if-else wrapper

Callbacks are expensive
Even vectorized callback functions degrades performace a bit
Alternative: implement ``callback'' in F77
Flexibility from the Python side: use a string to switch between the ``callback'' (F77) functions 
a = ext_gridloop.gridloop2_str(self.xcoor, self.ycoor, 'myfunc')
F77 wrapper: 
       subroutine gridloop2_str(xcoor, ycoor, func_str)
       character*(*) func_str
       ...
      if (func_str .eq. 'myfunc') then
         call gridloop2(a, xcoor, ycoor, nx, ny, myfunc)
      else if (func_str .eq. 'f2') then
         call gridloop2(a, xcoor, ycoor, nx, ny, f2)
      ...

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