Rover: cleanup mavlink_system setup
Plane: cleanup mavlink_system setup
AP_TECS: remove lag from height demand in landing

this predicts ahead the height demand for landing, where we have a
continuous demanded descent. This removes the effect of the lag
introduced by the height demand filters
AP_NavEKF: Do not reset vertical velocity state from GPS

Doing this can cause large height and height rate errors if large GPS velocity errors cause the GPS tn be rejected for long enough to cause a timeout and reset of states.
AP_NavEKF: Use named states for velocity reset

Readibility improvement
Copter: Fix LAND overshoot issue
Copter: minor formatting fixes
Mission: allow 15 do-jump commands on Pixhawk
AP_NavEKF: Improve robustness to long periods without GPS
AP_NavEKF: Reduce time required to recover from GPS timeouts

The time required for GPS to be lost or rejected before vehicles with airspeed sensors either reset to GPS or invoke the zero side-slip assumption to constrain drift has been reduced from 15 to 10 seconds
A duplicate zeroing of the GPS position offset has been removed
If the vehicle is a non hovering vehicle (eg a plane) then the speed at which the GPS offset is pulled back to zero after a reset is increased from 1 to 3  m/s

This also improves recovery from bad inertial data for planes
DataFlash: don't write out parameters if log open fails
DataFlash: don't try and open logfile on failure more than once

this prevents a corrupted microSD card from causing a continuous
attempt to open a log file while in flight, which can cause large
scheduler delays

Pair-Programmed-With: Grant Morphett <grant@gmorph.com>
Added Gitter badge
Notify: add pre_arm_gps_check flag

RGB LED will remain flashing blue when vehicle is disarmed and this
check has failed (i.e. false).
Copter: set pre_arm_gps_check flag
Plane: pre_arm_gps_check flag always true
Rover: pre_arm_gps_check flag always true
Tracker: pre_arm_gps_check flag always true
Notify: remove reliance on Led class
Notify: remove Led class

This class didn't add any functionality beyond what the NotifyDevice class
already provided so in keeping with our desire to keep things simple we
can remove this class