gsr: Added gsr sensor C source

Added the C source for the gsr sensor with necessary changes to cmake,
examples, docs.

    * Renamed all files with grovegsr to gsr
    * Replaced all instances of grovegsr with gsr
    * Added C source for gsr sensor
    * Updated all cmake files
    * Added C example for gsr sensor
    * Fixed C++ threshold method from throwing on a successfull aio read

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/gsr/pyupm_gsr.i

@@ -0,0 +1,11 @@
+// Include doxygen-generated documentation
+%include "pyupm_doxy2swig.i"
+%module pyupm_gsr
+%include "../upm.i"
+
+%feature("autodoc", "3");
+
+%include "gsr.hpp"
+%{
+    #include "gsr.hpp"
+%}