Externals Update, master branch (2019.02.27.)

This updates the master branch to use atlasexternals-2.0.23. The full list of changes wrt. atlasexternals-2.0.22 is: atlasexternals@2.0.22...2.0.23

Most importantly:

• Updated CheckerGccPlugins to avoid some false positives (@ssnyder, @fwinkl);
• Update to CLHEP to CLHEP_2_4_0_1_atl02 to fix the bug reported in CLHEP-154 (@mmuskinj);
• Updated AtlasCMake/AtlasLCG to use C++17 by default, with all compilers (@smh);
• Added nlohmann_parser as a new external (@rbianchi);
• Added VP1LightExternals, with some new external packages (@rbianchi);
• Updated AtlasLCG to provide find-modules for all the new python modules/externals introduced with LCG_95.

At the same time this MR updates Athena to use LCG_95. Note that atlasexternals-2.0.23 by default doesn't use LCG_95, Athena asks fr that version explicitly.

Note that the LCG_95 update is a continuation of !21425 (merged). With that merged in, the build against LCG_95 should succeed now. (I've played with this quite a bit during the last week.)

Finally, note that unfortunately there is still one weak point in all of this. 😦 Now that pytools, pygraphics and pyanalysis have been retired, the many find_package(pytools) etc. calls that we have in the repository are not doing anything anymore. Now, this does not affect the build. And when building the code by hand, it doesn't even affect the runtime environment. But once we try to install the built RPMs, we'll probably be in trouble. As the current setup of the Athena project will not install "enough" of the python packages of LCG.

This latter thing is unfortunately very painful to test for locally. But we may still want to hold off on merging this thing in until some further tests have been done.

Update:

After a first round of testing updated to atlasexternals-2.0.24 to fix the VP1 build error, and to include a fix for the "emacs issues" people have been seeing, into this MR. The full list of additional changes are: atlasexternals@2.0.23...2.0.24

Update 2:

In order to fix the missing cx_Oracle dependency, I now updated the MR to use atlasexternals-2.0.25, and added find_package(cx_Oracle) lines to a bunch of packages. (All that use cx_Oracle directly.)