Fix/2.1 awiesm 2.1 async icebergs

[ackerlar]

I removed the if clause for reading in the iceberg related namelist section in namelist.config. This enables to run icebergs also when compiled with ASYNC_ICEBERS=OFF. I also adapted the default nemlist.config.

[mandresm]

Looks good to me, @pgierz , do you agree?

[pgierz]

The default cmake list should have use icebergs off.

[ackerlar]

It does. I only changed the iceberg related code in a way that icebergs can be run in a non-asynchronous way if ASYNC_ICEBERGS is OFF.

[pgierz]

Sorry, I meant FESOM_COUPLED. I just fixed that; so, should be good to go. Green light from my side :-)

[pgierz]

OK sorry, red light: it looks like there are still somehow gfortran bugs :-(

51 |         call get_iceberg_nodes_for_element(mesh, localelement, ib_nods_in_ib_elem, num_ib_nods_in_ib_elem)
[187](https://github.com/FESOM/fesom2/runs/7688905736?check_suite_focus=true#step:4:188)
      |                                          1
[188](https://github.com/FESOM/fesom2/runs/7688905736?check_suite_focus=true#step:4:189)
Error: Explicit interface required for 'get_iceberg_nodes_for_element' at (1): target argument

[ackerlar]

Okay, I thought I did it with this commit here https://github.com/FESOM/fesom2/pull/363/commits/e1238a5b6888ae265c5f1bab8c5c07ff7be67ba7
But thanks! :)

[pgierz]

Looks like it got changed back in fe0a21e