Rewriting default flow connection according to the specification

[Adrian Pop]

This task investigates if the way we are dealing with unconnected flow connectors can be improved.
Right now the unconnected flow connectors are not saved in the connection sets as the Modelica
specification suggests, but a phases in which all flow components in the environment are checked
if they appear in the current connection set (if they don't appear a set-to-zero equation is generated).
An assessment of how much time this phase takes is needed to decide if the re-implementation according
to the specification will speed the compiler considerably. Note that this information (which flow is not
connected) is also required for finishing the implementation of Stream conenctors.

[Adrian Pop]

I implemented a cumulative timer in System.mo/systemimpl.c
For EngineV6.mos it reports:

Total time for default unconnected flow set-to-zero phase took: 3.34444652317353 seconds
out of:
real    9m43.842s
user    0m0.000s
sys     0m0.015s

So it seems the time for the phase in which we search for
all flow components that are unconnected is rather small: ~3.4 seconds out of ~9 minutes.

[Adrian Pop]

implemented in Revision: 6408