- put the rank and visited flags to SF_Block (protected side); this will render
  the SF_Network_Block_List_Content data structure obsolete
- attempt to remove the call to get_block_node from SF_Network::execute; this
  might have a drastic effect on performance
- have network::print "idle" on (rank==0) blocks
- are there problems with exceptions and resource (i.e. memory) freeing?
- check host_block in SF_Block::add_input ()
- SF_Network::is_functional () must check also for proper connections to input
  and output bubble terminals
- SF_Network::initialize () must check that all output bubble terminals are
  connected on the inside; it must also warn if some input bubble terminal
  isn't connected on the inside
- add flags to network in order to make it work more efficiently
- add flags (topology, relative sample rate) to SF_Network
- reduce redundancy, check whether e.g all partition_network(), is_functional(),
  propagate_sample_rates are necessary
- more exceptions could be derived from SF_Exception (e.g. math exceptions!)
- "index out of bounds" exception (SF_Indexing_Exception)
- SF_Terminal_Exception for errors related to a specific terminal instead of a
  whole block
- SF_Memory_Exception for out-of-memory situations
- put exception lists to all function declarations!
  e.g. void f() throw (X, Y); // throws ONLY X and/or Y and their descendants
       void g() throw (); // throws no exceptions
