cherry-pick PATInterfaces package from 21.2

Pretty straight-forward cherry-pick, plus adding the package to AnalysisBase.

Turns out the main difference were updates to copyrights, but some code changes were done as well.

List of commits involved:

• a64076b3 Fixing the package's build in standalone mode with CMake (PATInterfaces-00-00-35)
• afec45c1 Updated to PATInterfaces-00-00-38.
• c410de54 added test for SystematicSet clearing
• 9b309e93 Fixed SystematicSet clear behavior.
• a580600d boost::unordered_map --> std::unordered_map : ROOT-8890
• 54078d0b replace ISU copyright with CERN copyright
• ffdb051f make sure an empty string translates to an empty SystematicSet