Added PlainSecondsLogAdapter to parse simple lists of floats

Accapted example output of a benchmark harness is:

```
0.123123
0.2131221221
3.234323
error
```

As long as Python is able to parse the float it should be ok. One result by line, and results are expected to be in seconds.

Signed-off-by: Stefan Marr <git@stefan-marr.de>