Change sign in doppler computation

Based on mail conversation with Michael Stoehr and Cees Bassa. For
positive v_lsr, observable frequency should be lower than transmitted
frequency. See http://www.gb.nrao.edu/~fghigo/gbtdoc/doppler.html

This change makes that this doppler correction gives different outputs
than "Harm's scripts".
3 jobs for master
Status Job ID Name Coverage
  Build
canceled #101
build

 
  Test
canceled #103
allowed to fail
code_quality

canceled #102
test