[modules] geo_mag cleanup and doc improvements

conf/modules/geo_mag.xml

@@ -3,8 +3,9 @@
 <module name="geo_mag" dir="geo_mag">
   <doc>
     <description>
-      Geo Mag modell
-      by Sergey Kruskowski
+      GeoMagnetic field vector.
+      Calculation of the normalized geomagnetic field vector (saved to ahrs_impl.mag_h) at startup using GPS fix.
+      Based on the WMM2010 model (http://www.ngdc.noaa.gov/geomag/models.shtml).
     </description>
   </doc>
   <header>
@@ -12,10 +13,9 @@
   </header>
   <init fun="geo_mag_init()"/>
   <periodic fun="geo_mag_periodic()" freq="1"/>
-  <event fun="geo_mag_event()"/><!---->
+  <event fun="geo_mag_event()"/>
   <makefile>
     <file name="geo_mag.c"/>
-    <define name="USE_GEO_MAG"/>
     <file name="pprz_geodetic_wmm2010.c" dir="math"/>
   </makefile>
 </module>