Use interfaces for init_contiguous and init_zero_contiguous

After single-precision changes introduced in !98 (merged) , there are now procedure interfaces for:

  INTERFACE init_contiguous
    MODULE PROCEDURE init_contiguous_sp
    MODULE PROCEDURE init_contiguous_dp
    MODULE PROCEDURE init_contiguous_i4
    MODULE PROCEDURE init_contiguous_l
  END INTERFACE init_contiguous

  INTERFACE init_zero_contiguous
    MODULE PROCEDURE init_zero_contiguous_dp
    MODULE PROCEDURE init_zero_contiguous_sp
  END INTERFACE init_zero_contiguous

Todo - replace subprocedure calls with interface calls in ICON. Then make just the interface public in fortran-support.

mentioned in merge request !98 (merged)