AC_Fence: A circular fence with a radius of 0 meters is ignored

[muramura]

I am developing an application for vehicles that support MAVLINK messages. I have discovered different behaviors in the setting of circular fences. I want to ignore circles with a radius of 0 meters. By ignoring circles with a radius of 0 meters, I can standardize the processing with vehicles other than ArduPilot.

AFTER

BEFORE

[rmackay9]

I guess the advantage of supporting this is that it allows an external system to essentially disable a fence without removing it from the list.

The downside is that it introduces an edge case that we will need to be careful of in the flight code. The flight code may or may not be ignoring circular fences with zero radius.

TBH, I'm not sure it's a good idea to make this change. External systems should be able to enable/disable in other ways which don't increase the complexity of the flight code.

[muramura]

@rmackay9 san.
I'm modifying the process for adding circle fences so that only those with a positive radius are registered.

[peterbarker]

I'm really confused by this - we're enforcing different rules for the fence in different places, which is just really rather odd.

And patching in "write_fence" is very strange.