Have the Session object provide methods for retrieving information about other applications

[eflumerf]

Several SmartDaqApplications need information about other applications in the Session to set up network connections. (e.g. https://github.com/DUNE-DAQ/appmodel/blob/f0eee9340ab38266abaed96d6a85d16d9b22d1e9/src/DFApplication.cpp#L261)

The Session object should be able to provide this information, so that SmartDaqApplications (and DAQModules) can remain ignorant of the state of the rest of the system.

[gcrone]

I think this really needs to have NetworkRule and SmartDaqApplication in the confmodel package. This is not feasible while we are using the current hack to call the appropriate get_modules() method in SmartDaqApplication. It should be OK if/when we start using the thea/virtual_strikes_back branches.