This should avoid the awkward handling.  The map makes sure that only the
central frequencies are passed around which have been used in the observation.

Also be nice an handle exceptions thrown by DAL.

SW-488: used tbb_freeze as a test routine for the new lcurun like execute_in_parallel_over_stations methods, which use the new execute_in_parallel methods

I am afraid that the 0Hz central frequency can be attributed to an integer
divided by a double.  This should at least remove the uncertainty.

SW-488: only use cep4 nodes where no datawriters are running, and ask politely for a minimum amount of nodes (not guaranteed)

SW-488: minor fixes, logging, and added a parameter 'min_nr_of_nodes' to get_cep4_available_cpu_nodes_sorted_ascending_by_load so we can always get at least a number of cpu nodes, even if they are under (too) heavy load

Also refactored some functions out of the TBB_Dipole clase since they are
not essential fot the object itself.

Task SW-560: Remove unnecessary bash -c from call, which provoked a syntax error. We still have to escape the bash variables in the remote script.