Restrict flows to Paris and Dublin strategies

[fujiapple852]

The calculation and display of flows (including dot and flows reports) should be restricted to Paris and Dublin strategies only.

Each probe sent in each round is an "independent trail" and may follow a different network path and therefore produce flows which do not exist.

Using the Paris and Dublin tracing strategy all probes within a round are more likely (though not guaranteed) to follow the same path and therefore produce accurate flows. When operating with FixedSrc port (default for UDP) or FixedDest port (default or TCP) for the Paris or Dublin strategy, the variable port is constant for all probes sent within a single round.

For example, for UDP (Paris or Dublin) with a fixed src port (5000) and a variable dest port and initial sequence of 33000, the ports used for each probe will be as follows:

Round	Hop (ttl)	SrcPort	DestPort	Seq
0	1	5000	33000	33000
0	2	5000	33000	33001
0	3	5000	33000	33002
1	1	5000	33001	33003
1	2	5000	33001	33004
1	3	5000	33001	33005
2	1	5000	33002	33006
2	2	5000	33002	33007
2	3	5000	33002	33008

As the src and dest port are constant within each round, the path through the network is more likely to be the same for each probe within that round and therefore the flow calculated for the round is more likely to be genuine.