Utils.metrics added functionality

(This is a remake of #200 but getting rid of those 2 extra commits that sneaked into it by accident)

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.