Added the C source for the flex sensor with necessary changes to cmake,
examples, and docs.
* Renamed all files with groveslide to slide
* Replaced all instances of groveslide with slide
* Added C source for slide sensor
* Updated all cmake files
* Added C example for slide sensor
* Split out slide sensor from grove library
Signed-off-by: Noel Eck <noel.eck@intel.com>