feat: speed up CI builds with unity build

[wdconinc]

Briefly, what does this PR introduce?

To speed up CI builds, we can use unity builds, essentially bundling multiple source files in a single source file per library, and compiling in one go. This speeds up a non-ccache compilation locally from

        Elapsed (wall clock) time (h:mm:ss or m:ss): 6:37.68

to

        Elapsed (wall clock) time (h:mm:ss or m:ss): 4:15.68

(Methodology: rm -rf build && cmake -Bbuild -S. -DCMAKE_UNITY_BUILD=ON -DCMAKE_UNITY_BUILD_BATCH_SIZE=24 --fresh && /usr/bin/time -v make -Cbuild -j8 install)

Unity builds are not without their drawbacks. It requires that the different source files can coexist in a single source file. They cannot define functions with the same name in the same namespace. It is in general a good practice to avoid these conflicts (by defining them once), so in addition to the benefit of faster compilation, this is a useful for the code base quality.

Needs:

• feat: add check for presence of pragma once in headers #2058
• fix: remove use of JANA/CLI/JVersion.h #2059
• fix: remove use of operator-> for podio collection objects #2060
• fix: remove duplication of edm4eic_indexed_units in src/algorithms/tracking #2062

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue #__)
• Documentation update
• Other: __

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?

No.

Does this PR change default behavior?

No.