Factor out _calc_mean() and _calc_median() functions and call from
both run_probe() and cmd_PROBE_ACCURACY().
This also fixes a subtle error in the run_probe() median function - on
some kinematics the x and y position can change on a z move so the x
and y should be taken from the z probe values actually used.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>