symbol correction

[gooyle]

[gooyle]

thx for your package！！！it helps a lot,i think here are some mistakes, i use xsens data to test.
in file:

ahrs/ahrs/filters/mahony.py

#Rotate magnetic field to inertial frame
v_m = [e-mail]([-np.linalg.norm([h[0], h[1]]) , 0.0, h[2]]) ##no minus
v_m = [e-mail]([np.linalg.norm([h[0], h[1]]) , 0.0, h[2]])
and here is the results，shown in roll、pitch and yaw：

• 
  and here is the results about m and v_m:
  

[Mario Garcia]

This was adressed already in the commit 34f959f, and there was a major change with it too.

The creation of the v_m was, apparently, not correct. Please, test the new version, if possible, and let me know if you see any improvements at all.