Externals Update, master branch (2019.10.10.)

Updated all projects to atlasexternals-2.0.41. The full list of changes (atlasexternals@2.0.40...2.0.41) is:

• Taught AtlasLCG how to find system-installed python modules correctly;
• Added fixes for the optional installation of components in projects. (To make the installation succeed even if the build didn't.)
• Now using FindPython and FindPython2 for finding Python in all projects;
• Added Python 3 compatibility fixes to the Gdb and PyModules packages, and to AtlasLCG modules;
• Added an option for atlas_install_python_modules(...) not to fail on syntax errors in the installed files;
• Moved the reading of the project specific PreConfig.cmake files earlier in the CMake configurations of those projects, to use flake8 correctly in the WorkDir project.

Almost all of these are meant for the Python 3 migration discussed in ATLINFR-3221. Apart from @fwinkl's update specifically meant for flake8. 😉

Note that I also updated VP1Light to this latest version of the externals. I believe it should be fine, but @rbianchi should speak up if not.