Add module to support different autopilots

[hmakelin]

Add support for ArduPilot, see also https://github.com/hmakelin/gisnav-docker/pull/12

• Adds new module that defines an interface for supporting different autopilots.
• Adds elementary support for static (relative to vehicle body) cameras. Should probably later move to a separate module so that support for static cameras and gimbals with varying degrees of freedom could be implemented properly.
• Mock GPS node GPS message is published directly over MAVLink because of earlier communication issues with mavros node. Better solution would still be to use the mavros GPS_INPUT ROS topic instead if it works.
• Bump up version number to 0.61.0.

[hmakelin]

Update: Will also include handling of static camera in this PR since the planned ArduPilot SITL simulation will have a camera that is assumed static relative to vehicle body. Earlier versions implicitly assumed 2-axis gimbal (stabilized pitch and roll).