Test Harness Gnd dist fixes

@LorenzMeier:
  * 341c5b2ec6742ddd3a08e6b303f7351d98d58276 makes the filter only use valid data, this is consistent with this onboard check: https://github.com/PX4/Firmware/blob/fwlandingterrain/src/modules/ekf_att_pos_estimator/ekf_att_pos_estimator_main.cpp#L1154 Due to this changes the test harness seems now to use the laser (--> update the ground estimate) at the same moments as the onboard version.
  * da79e1d271fd6a629e94145218ba6dc077247a84: makes the ground estimate calculated from the laser (which we plot for comparison only) look the same as onboard. The raw sensor value is used independent of the valid flags.