python: Build both python2 and python3 modules

Updated UPM cmake to build both python2/3 modules if the necessary
python libs are available.

    * Removed BUILDPYTHON3 cmake option in favor of building both
      versions
    * Added cmake module to find both python2 and python3
    * Added macro which can be called from per-sensor cmake files
      to add dependency libraries via target_link_libraries as well
      as swig_link_libraries (for java, nodejs, and python)
    * Python2 is required for python documentation builds
    * Cleanup of python usage throughout UPM cmakelists.
    * Make find_package for Node required.  Removed check for NODE_FOUND
      (which should have been NODEJS_FOUND) in favor of REQUIRED.
    * The cxx and .py wrapper files get generated in python2/3 (instead
      of the same directory).  It appears these files are identical,
      however since the targets can be built in parallel it's safer to split
      them out.
    * Updated all cmake dependencies related to building documentation.
    * Removed unused classname from cmake files

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

src/lcd/CMakeLists.txt

@@ -1,5 +1,4 @@
 set (libname "i2clcd")
-set (classname "lcd")
 set (libdescription "upm lcd/oled displays")
 set (module_src lcd.cxx lcm1602.cxx jhd1313m1.cxx ssd1308.cxx eboled.cxx ssd1327.cxx sainsmartks.cxx ssd1306.cxx)
 set (module_hpp lcd.hpp lcm1602.hpp jhd1313m1.hpp ssd1308.hpp eboled.hpp ssd1327.hpp ssd.hpp sainsmartks.hpp ssd1306.hpp)