doxygen: add groups for all modules

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/hmc5883l/hmc5883l.h

@@ -29,6 +29,11 @@
 
 namespace upm {
 
+/**
+ * @brief HMC5883l magnometer library
+ * @defgroup hmc5883l libupm-hmc5883l
+ */
+
 /**
  * @brief C++ API for HMC5883l (3-axis digital compass)
  *
@@ -38,10 +43,10 @@ namespace upm {
  * all done through an I2C interface. Different breakout boards are available,
  * typically a 3V supply is all that is needed to power the sensor.
  *
+ * @ingroup hmc5883l
  * @snippet hmc5883l.cxx Interesting
  * @image html hmc5883l.jpeg
  */
-
 class Hmc5883l {
 public:
     /**