WIP: Include lwtnn in Athena

Hi @akraszna,

It would be great if I can get some help on this...

This MR adds lwtnn to Athena, specifically, so it can be used in packages like AnalysisTop and BoostedJetTaggers:https://gitlab.cern.ch/atlas/athena/merge_requests/3085. There are some questions:

• Is AnalysisBaseExternals the correct place to include lwtnn so that it gets found/used by both above mentioned packages?
• lwtnn only needs Boost and Eigen headers. However, it's not a bad idea to require the full package be built just for simplicity and the fact that it will happen anyway. However, I'm not finding the correct Boost headers in the way that I've done it in the CMakeLists.txt file. Maybe you can comment?

If in my environment I include something like

localSetupSFT -q releases/LCG_84/eigen/3.2.7
localSetupBoost -q boost-1.60.0-python2.7-x86_64-slc6-gcc49
EIGEN_INC=$SFT_HOME_eigen/include/eigen3
BOOST_INC=$ALRB_BOOST_ROOT/include
export CPLUS_INCLUDE_PATH=$EIGEN_INC:$BOOST_INC

Then the package gets build all hunky-dory. So I just need to translate the above into Atlas cmake.