The CPU backend is disabled by default and must be enabled with
the CMake variable NGRAPH_CPU_ENABLE set to true

- Adds Sphinx docs support.

- Sphinx and Doxygen docs now built by CI.

- Sphinx and Doxygen docs now installed by `make install`.

Git invocations without an available gitconfig (ex. a test environment)
will fail whereas patch should always work. This adds a build dependecy on patch.

Currently MKL-DNN isn't supported on this platform

add test framework

stuff wired up

benchmark segfault for 5M nodes. fun

make traverse_nodes not recursive

print timings on benchmark

unit test for pass manager

This is temporary and will be removed once upstreamed

This was removed because the previous eigen CMake spec forced a new CMake version
but that was taken out in a previous commit

add cmake finder for graphviz dot program and then conditionally compile visualization based on finding

add eigen as external project.
add very simple eigen unit test to test building/linking

Disable infinite recursion in ScalarConstant::node_id()

remove comment out line per review comments