update to accel calibration screen needed

I've just had a very frustrating experience with a user using the EKF in APM:Plane. They were sure they were doing a 3D accel calibration, but the logs showed 1.0 exactly for the accel scaling. It turns out they were doing both the 3D and the 1D accel calibration.
Looking at the MissionPlanner accel cal screen I can see why - it shows both accel calibration methods and doesn't make it clear that the user should do just one or the other.
I'd suggest making it very clear that only one of them should be done. I also think that if AHRS_EKF_USE=1 is set then the user should get a warning if they do a 1D calibration. Something like "WARNING: 1D accel calibration is not recommended when the EKF is enabled. Please do a 3D accel calibration instead"
Thanks!

Start by locating the MissionPlanner accelerometer calibration screen and the handling for AHRS_EKF_USE. Review how the 3D and 1D calibration options are presented and how warnings are displayed. Done means the screen clearly directs users to choose one method and warns against 1D calibration when the EKF is enabled.

Written by the indexing model from the issue text.