Analysis Release LCG_100 Updates, master branch (2021.05.10.)

With the "non-Analysis" releases moving to LCG_100_ATLAS_1 in athena!43271 (merged), it's time to update the analysis releases as well. 😉

This MR does the following:

• Introduces simple patches for KLFitter and BAT to make them compatible with ROOT 6.24/00;
• Upgrades Boost to version 1.75.0;
• Upgrades HDF5 to version 1.10.6;
• Upgrades libxml2 to version 2.9.10;
• Upgrades nlohmann_json to version 3.9.1;
• Upgrades Cython to version 0.29.21 and pip to version 21.0.1 (both as part of the PyAnalysis package);
• Upgrades XRootD to version 5.1.1;
• Upgrades ROOT to version 6.24/00.

I'm still running some of my own tests with the update (on various OS-es used during analysis). Until their conclusion the MR will remain open.