doxygen: add groups for all modules

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

src/gas/gas.h

@@ -36,9 +36,8 @@ struct thresholdContext {
 namespace upm {
 
 /**
- * @brief C++ API for Gas sensors
- *
- * This file defines the Gas Analog sensors virtual functions
+ * @brief gas sensor library
+ * @defgroup gas libupm-gas
  */
 class Gas {
     public:

src/gas/mq2.h

@@ -36,6 +36,7 @@ namespace upm {
      * Hydrogen, smoke and other combustible gases. It's a medium sensitivity
      * sensor with a detect concentration of 300-10000ppm.
      *
+     * @ingroup gas
      * @snippet mq2-example.cxx Interesting
      * @image html grovegas.jpeg
      */

src/gas/mq3.h

@@ -36,6 +36,7 @@ namespace upm {
      * highly sensitive but has a long warmup time of about 1minute. It's
      * detect rate is of 0.04-4mg/L Alcohol.
      *
+     * @ingroup gas
      * @snippet mq3-example.cxx Interesting
      * @image html grovegas.jpeg
      */

src/gas/mq5.h

@@ -36,6 +36,7 @@ namespace upm {
      * on. It is highly sensitive and has a detection concentration of
      * 300-10000ppm.
      *
+     * @ingroup gas
      * @snippet mq5-example.cxx Interesting
      * @image html grovegas.jpeg
      */

src/gas/mq9.h

@@ -35,6 +35,7 @@ namespace upm {
      * home and industry). It can detect Carbon Monoxide, Coal Gas and
      * Liquefied Gas. It's sensitivty is 10-1000ppmCO 100-10000PPm Gas.
      *
+     * @ingroup gas
      * @snippet mq9-example.cxx Interesting
      * @image html mq9.jpeg
      */