* add nop backend

* nop backend

* fix flag name

* add new switch to cmake output of switch settings

* add new unit test to igpu manifest

* remove redundant test

* Update "JSON for Modern C++" to a version that officially supports gcc 4.8

* "JSON for Modern C++" is header only and does not need to be configured.

* move backend specific test files to test/backend directory

* remove unit_test_control

* move tests back to test root

* fix comment

* wip

* fix manifest

* backend libraries now found in tree

dynamically read header search paths

fix running from install

* Add mechanism for disabling specific backend unit tests from a manifest file.
Populate the test manifest files for CPU, GPU and INTERPRETER.

* update docs for new manifest controlled transformer unit tests

* migrate files

* apply version number to libngraph.so

* fix cmake running with empty build directory

* patch working

* wip

* fix patcher

* remove debug message:

* cleanup

* fix typo

* add support for both command line arg and env var to set NGRAPH_WARINGS_AS_ERRORS'

* Add mention of blob ref of original file from caffe2

* Mention location of source listing originally from LLVM project

Reused from Caffe 2 (Apache Licensed)

[Link](https://github.com/caffe2/caffe2/blob/master/cmake/Modules/FindCuDNN.cmake)

- Adds Sphinx docs support.

- Sphinx and Doxygen docs now built by CI.

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

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