Patch: Running example_vasarhelyi with quadcopter drone type

[d-marson]

These proposed changes are based on attempts to run example_vasarhelyi with DRONE_TYPE = "quadcopter"

First issue addressed:

• Indexing error occurs on line 287 of Swarm.m after the main simulation is finished but during plotting and swarm state analysis. The error seems to be because vel_xyz_history is empty. When example_vasarhelyi is run with DRONE_TYPE = "point_mass", each drone's pos_ned_history and vel_xyz_history are saved by the method Drone.update_state (lines 376 to 387). When running DRONE_TYPE = "quadcopter", Drone.compute_kinematics is called instead and it saves pos_ned_history but does not save vel_xyz_history.
• This merge request suggests a modification to Drone.compute_kinematics to save vel_xyz_history. The if statement needs to check for an empty array because vel_xyz_history does not have values assigned to it prior to this point (pos_ned_history appears to have values assigned to it by the method Drone.set_pos by swarm initialization)
• I have also encapsulated the check against a vector of NaNs within an isequal to ensure the if statement does its check against a logical scalar.

Second issue addressed:

• The drones appear to spend the first part of the simulation traveling down, below an altitude of 0, before they start to maintain altitude. The file param_swarm includes a line that is commented out with the comment "leave empty if you use a real drone model."
• The addition of an if statement that assigns a value to p_swarm.max_a based on DRONE_TYPE appears to fix the issue, and is consistent with other usages of DRONE_TYPE in other parameter files to assign parameter values.
• See attached plots.