Switch to factory functions, style on some more files.

Bob/element type

update broadcast op args

Inverse arguments, missing file in last PR

Cyphers/view

Add contrib/docker, for reference build environment and initial CI

tweak cmake settings to work better on mac

remove comment out line per review comments

Initial implementation of the plugin library

Added "make check_cpu" target.  Removed unused definitions from Makefile.  Added chown_files.sh to ngraph_cpp_cpu image, instead of creating new image.