updating based on review

Updated with feedback from @silee2

Edits to address @lnguyen-nvn comments.

Signed-off-by: Evan Couzens <evan.couzens@intel.com>

- Adds Sphinx docs support.

- Sphinx and Doxygen docs now built by CI.

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

This adds support for > < >= <= on bool, which by the type checker's standards should have been allowed anyway.

Also, this bug had presenting in a different way from expected (should have thrown "unhandled element type" from external_function, instead bonked because it incorrectly dispatched on int8_t). This was due to element::Type::operator== comparing only size, signedness, and floatiness, meaning that char and signed char (aka int8_t) look the same. Fixed by adding a test for m_cname equality.

Bob/test cleanup

make CMake report if gcc4.8 is being used

NGRAPH_INSTALL_PREFIX instead of CMAKE_INSTALL_PREFIX

Basic autodiff for +, -, *, /, exp, log, plus numeric testing support

* working unit test currently fails as expected

* unreferenced args unit test pass

* cleanup

* cleanup

Multiply and Add numeric/autodiff unit tests pass

Documentation for all ops that have been defined so far. Includes numerous edits by @ECouzens.

* Refactored to run builds as user, not as root.  Added make install.  Added parallel build.  Expanded make clean to remove BUILD directory.  Moved log generation to makefile, so Jenkins job can let make do all of the work.

* Changes/fixes to address code review.  Mainly adding double-quotes around variables and using $(...) expansion where appropriate (and semantically identical to back-quotes).

* Add double-quotes for variable expansion safety.  Write error message in run_as_user.sh to stderr.

* Remove chown_files.sh, made obsolete by running as user inside docker container.  Add set -e and set -u to run_as_user.sh, for further protection.