solo-1.4.0

diff from 1.3.8:

• GPS failsafe behavior has been re-worked significantly. One of the main takeaways is that minor GPS glitches will not result in ALT_HOLD. Catastrophic GPS failure will result in LAND mode.
• Improved landing detection by reducing lean limit.
• Improved motor mixer to increase thrust margin (important for high altitude operation with heavy payload)
• Reduced pre-arm false positives on surfaces with large magnetic anomalies by passing the check after 3 consecutive seconds of compass consistency.
• Removed gyro calibration on arming to prevent "Motion Detected" messages.