AP_Compass: make get_field() and get_offsets() return milligauss

This is part of a transition to make AP_Compass always use a single unit for
magnetic field values, namely milligauss.

libraries/AP_Compass/Compass.h

@@ -119,9 +119,9 @@ public:
     uint8_t get_count(void) const { return _compass_count; }
 
     /// Return the current field as a Vector3f
-    const Vector3f &get_field(uint8_t i) const { return _state[i].field; }
+    const Vector3f &get_field(uint8_t i) const { return _state[i].field * _state[i].milligauss_ratio; }
     const Vector3f &get_field(void) const { return get_field(get_primary()); }
-    const Vector3f get_field_milligauss(uint8_t i) const { return get_field(i) * _state[i].milligauss_ratio; }
+    const Vector3f get_field_milligauss(uint8_t i) const { return get_field(i); }
     const Vector3f get_field_milligauss(void) const { return get_field_milligauss(get_primary()); }
 
     // raw/unfiltered measurement interface
@@ -182,9 +182,9 @@ public:
     ///
     /// @returns                    The current compass offsets.
     ///
-    const Vector3f &get_offsets(uint8_t i) const { return _state[i].offset; }
+    const Vector3f &get_offsets(uint8_t i) const { return _state[i].offset * _state[i].milligauss_ratio; }
     const Vector3f &get_offsets(void) const { return get_offsets(get_primary()); }
-    const Vector3f get_offsets_milligauss(uint8_t i) const { return get_offsets(i) * _state[i].milligauss_ratio; }
+    const Vector3f get_offsets_milligauss(uint8_t i) const { return get_offsets(i); }
     const Vector3f get_offsets_milligauss(void) const { return get_offsets_milligauss(get_primary()); }
 
     /// Sets the initial location used to get declination