Fix bug with multiple updates to adjoint map

- 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.