build: use GNUInstallDirs and improve packaging

[c-dilks]

Briefly, what does this PR introduce?

• Use GNUInstallDirs to set the installation tree, instead of doing it "by hand"
• Install cmake files in lib/cmake/IRT instead of in lib/IRT
• Add public-facing variables IRT_LIBRARY_DIR and IRT_INCLUDE_DIR
• This requires updates in EICRecon: build: use IRTConfig.cmake vars EICrecon#546

Installation tree before this PR:

CMAKE_INSTALL_PREFIX
├── include
│   └── IRT
│       ├── BitMask.h
│       ├── ChargedParticle.h
│       ├── ChargedParticleStep.h
│       ├── CherenkovDetectorCollection.h
│       ├── CherenkovDetector.h
│       ├── CherenkovEvent.h
│       ├── CherenkovMirror.h
│       ├── CherenkovPhotonDetector.h
│       ├── CherenkovPID.h
│       ├── CherenkovRadiator.h
│       ├── G4Object.h
│       ├── IRT.h
│       ├── IRTSolution.h
│       ├── OpticalBoundary.h
│       ├── OpticalPhoton.h
│       ├── ParametricSurface.h
│       ├── RadiatorHistory.h
│       ├── ReflectionPoint.h
│       ├── RefractionPoint.h
│       ├── SinglePDF.h
│       └── TransientParticle.h
└── lib
    ├── IRT
    │   ├── IRTConfig.cmake
    │   ├── IRTConfigVersion.cmake
    │   ├── IRTTargets.cmake
    │   └── IRTTargets-noconfig.cmake
    ├── libDELPHES_rdict.pcm
    ├── libDELPHES.rootmap
    ├── libDELPHES.so
    ├── libIRT_rdict.pcm
    ├── libIRT.rootmap
    └── libIRT.so

After this PR:

CMAKE_INSTALL_PREFIX
├── include
│   └── IRT
│       ├── BitMask.h
│       ├── ChargedParticle.h
│       ├── ChargedParticleStep.h
│       ├── CherenkovDetectorCollection.h
│       ├── CherenkovDetector.h
│       ├── CherenkovEvent.h
│       ├── CherenkovMirror.h
│       ├── CherenkovPhotonDetector.h
│       ├── CherenkovPID.h
│       ├── CherenkovRadiator.h
│       ├── G4Object.h
│       ├── IRT.h
│       ├── IRTSolution.h
│       ├── OpticalBoundary.h
│       ├── OpticalPhoton.h
│       ├── ParametricSurface.h
│       ├── RadiatorHistory.h
│       ├── ReflectionPoint.h
│       ├── RefractionPoint.h
│       ├── SinglePDF.h
│       └── TransientParticle.h
└── lib
    ├── cmake
    │   └── IRT
    │       ├── IRTConfig.cmake
    │       ├── IRTConfigVersion.cmake
    │       ├── IRTTargets.cmake
    │       └── IRTTargets-noconfig.cmake
    ├── libDELPHES_rdict.pcm
    ├── libDELPHES.rootmap
    ├── libDELPHES.so
    ├── libIRT_rdict.pcm
    ├── libIRT.rootmap
    └── libIRT.so

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue #__)
• Documentation update
• Other: refactor the build config

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

Yes, see above PR on EICrecon. This also might break Juggler's usage (on a dev branch, not main).
This also might impact @alexander-kiselev's usage

Does this PR change default behavior?

No, but hopefully it solves our various build/linking/cling/etc. problems in EICrecon...

[c-dilks]

@veprbl and @wdconinc, I'm still testing this, but could you take a look to see if what I'm doing makes any sense? I am still fairly inexperienced with cmake configuration and would greatly appreciate any advice.

[c-dilks]

Tested IRT PID in EICrecon, still works fine.

[veprbl]

LGTM