BUG: Calculate ruler distance so it's up to date

When adding a ruler node programatically to the scene, with initialised
end point coordinates, the distance wasn't getting updated. Removed the 
local variable that saved the distance and changed GetDistanceMeasurement
to calculate it when called. Still writes out the distance to MRML
but does not read it back in as it will be re-calculated.

Issue #3123



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22005 3bd1e089-480b-0410-8dfb-8563597acbee