Add a Date Range duration formatter

Problem/Motivation

I need i my project to display time between two dates, but I must use a DateRangeField to set it.
Sometimes i must display a string like : 3 hours 15 minutes or
in decimal like 3.25h
to make some calculations after in a views.

Proposed resolution

I created this field formatter (for the last 8.x-1.x-dev) and let's you have a try...
I will work after on the "test" part to maybe be one day become a core part !

Regargs
Jo