OPEN - task 665: Generate forecast Doppler between two tracks 
http://github.com/debrief/debrief/issues/issue/665

First working calculation.
introduce ability for a drop-down calculation to have its own wizard
introduce better text, plus nice image
only allow relative calcs if there is more than one track selected
make the calculation get/set double values, not strings
refactoring, to allow a wizard to be specified
Merge pull request #867 from debrief/issue665_doppler

Issue665 doppler