vprof: use VP_OMEGA

Summary:
use VP_OMEGA

 * replace string constants with constant variables
 * use VP_OMEGA instead of vp_upward_air_velocity_ml, do hack/scaling in detup file
 * simplify code in some places
 * try to avoid some memory leaks

Test Plan: run vprof in diana

Reviewers: #diana, audunc

Reviewed By: #diana, audunc

Subscribers: YngveE

Differential Revision: https://phab.met.no/D33