Add get_accel_ef_blended function to AP_AHRS

- Vibration problems could cause a climb on copter because we were directly using the primary accelerometer for feedback acceleration control.
- DCM problems could cause issues with altitude control even with EKF running, because DCM's matrix is used to update _accel_ef.
This patch corrects both issues, adds a get_accel_ef_blended function to AP_AHRS.