Utils.metrics added functionality

2 patches changes in this pull request:

a) states.PerfdataMetric.split_value_and_uom() moved to be a module level function instead of a method of PerfdataMetric

b) new function get_base_value() designed for common arithmetic like convert gigabytes into bytes.
I'm not sure this is quite right..

Here is my read into it:
```
(
   [-]*       dash - zero or more times
   [\d.]*     digit or anything - zero or more times
   \d+        digit - once or more times
)
(
   .*         anything - never or more times
)
```

I think we should test more variants, like floating point numbers.