api to simplify iperf result retrieval

Currently iperf just returns parsed output as dict

Would be great to have API like
```python
throughput =iperf.final_bandwith(unit="Mbits/sec")
transfered =iperf.total_transfer(unit="MBytes")
```
It should take last record of report (the one with interval 0-max) and recalculate bandwith according to requested unit.