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/ms5611/pyupm_ms5611.i

@@ -1,5 +1,7 @@
 %module pyupm_ms5611
 %include "../upm.i"
+// Include doxygen-generated documentation
+%include "pyupm_doxy2swig.i"
 
 %feature("autodoc", "3");