added support in iperf for different result formats. 

also added upper bounds for hifi tests. There were also a few small errors in the tests including:

- didn't take arp into account on link delay test
- needed to convert from percentage to decimal on cpuLimit test
- UserSwitch was not fast enough to have a 5Mbps bandwidth, so changed test to .5 Mbps

fixes #231
fixes #215
This isn't quite right. The default iperf behavior is to pick a reasonable format given the bandwidth, and we probably want this to continue to be the default.

Here we assign 'M' (MBytes/s) as the default value, but then we fail to set that, even if the user explicitly requests it.

We should probably set the default to 'None' and then just say 'if format:'