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>