Provide isReadyToFly boolean instead of waiting for takeoff to finish.
It is now up to the user to check if the drone is ready to receive commands.

See merge request !1

Checking for landing is used for a security purpose. If landing procedure is
triggered but not by the swarm management script, it means it has been triggered
externally most probably for emergency reason.

Altitude should not be overriden with default value as it is defined without
knowing the field or the flight conditions. Therefore the user has to provide
coordinates with the desired altitude so altitude override can happen.

Set default altitude override to 30 meters above ground.
Set default airspeed to 17 meters per second.

Do not do any projection before target coordinates have been set.

Initialize takeoff altitude at DEFAULT_OVERRIDE_ALTITUDE meters above ground.

Stop projecting destination

Fetch position from GLOBAL_POSITION_INT to get timestamped positions

Getters removed because these data should be fetch through position array.

Use position messages instead of gps ones in case of gps denial.

Now `setTargetCoordinates` always go through the desired point by updating
regulary the projected coordinates.
Function `loiter` is now defined for the purpose where `setTargetCoordinates`
was called with `going_through` parameter set to `false` (to loiter around a
point without going through it).