AC_WPNav: ACCEL_C defaults to 2x ACCEL

[rmackay9]

We've had numerous support issues involving users complaining that Copter-4.4 doesn't cut the corners as much as is it in previous versions. The advice to users has been to set WPNAV_ACCEL_C to 2x the WPNAV_ACCEL value to get back to the behaviour they had in previous versions. This PR makes the default WPNAV_ACCEL_C produce this behaviour.

This changes the maximum cornering acceleration to be 2x the regular waypoint acceleration (previously was 1x).

Increasing this acceleration results in the WPNAV_RADIUS parameter being the most likely limitation to how widely SCurves corners are cut.

Below are before-vs-after screen shots of the path taken during a rectangular mission with WPNAV_RADIUS = 5000 (e.g. 50m).

Here are before-vs-after screen shots when WPNAV_RADIUS = 200 (e.g. 2m which is the default) and we see there's no obvious change in behaviour.

[lthall]

Yep, I think this is the right thing to do. I went with the conservative approach first but this gives the most efficient flight profile.

[andyp1per]

Candidate for 4.4 maybe?

[amilcarlucas]

This is probably the only 4.4 regression. So I would say yes.

[rmackay9]

Yes, good idea. I've added it to the 4.4 project and I suspect we will have a 4.4.5 soon.

[rmackay9]

OK, two approvals so in it goes!