make CMake report if gcc4.8 is being used

Remove commented-out code and predicate debuginfo generation

Also disable PCH in preparation of merge

via op::ParameterizedConstant

NGRAPH_INSTALL_PREFIX instead of CMAKE_INSTALL_PREFIX

Also make Eigen format construction explicit

The notion of an instruction will probably come back
but with a different implementation.

This reduces compilation time by ~10x

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

* working unit test currently fails as expected

* unreferenced args unit test pass

* cleanup

* cleanup

that will be incrementally replaced