doxygen: add groups for all modules

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

src/max31855/max31855.h

@@ -29,6 +29,11 @@
 
 namespace upm {
 
+/**
+ * @brief max31855 thermocouple library
+ * @defgroup max31855 libupm-max31855
+ */
+
 /**
  * @brief C++ API for MAX31855
  *
@@ -40,6 +45,7 @@ namespace upm {
  * (http://datasheets.maximintegrated.com/en/ds/MAX31855PMB1.pdf) from the
  * analog PMOD kit.
  *
+ * @ingroup max31855
  * @snippet max31855.cxx Interesting
  * @image html max31855.jpeg
  */