Orderpack: thread safe

The module mo_orderpack was not thread-safe since it was using a global variable IDONT in the indmed subroutines (index of median).

This merge request makes this variable local in all routines and marks these routines as recursive so local variables will be created for each thread when the method is called with OpenMP.

Actually, the Fortran 2018 standard makes recursive the default mode for functions and subroutines, but older gfortran versions failed in combination with OpenMP.