doxygen: add groups for all modules

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

src/max31723/max31723.h

@@ -33,6 +33,11 @@
 
 namespace upm {
 
+/**
+ * @brief max31723 temperature sensor library
+ * @defgroup max31723 libupm-max31723
+ */
+
 /**
  * @brief C++ API for MAX31723 chip (Temperature sensor)
  *
@@ -43,6 +48,7 @@ namespace upm {
  * module](http://datasheets.maximintegrated.com/en/ds/MAX31723PMB1.pdf) from
  * the analog PMOD kit.
  *
+ * @ingroup max31723
  * @snippet max31723.cxx Interesting
  * @image html max31723.jpeg
  */