Add RMC phase-space approximation fcl

[michaelmackenzie]

This is connected to Offline PR: Mu2e/Offline#1879. This adds fcl to generator the 0 and 1 neutron knockout RMC spectra. These can then be used in mu- --> e- and mu- --> e+ analyses using the branching fractions measured in recent studies.

[FNALbuild]

Hi @michaelmackenzie,
You have proposed changes to files in these packages:

• Tests
• JobConfig

which require these tests: build.

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⌛ The following tests have been triggered for a506120: build (Build queue - API unavailable)

About FNALbuild. Code review on Mu2e/Offline.

[FNALbuild]

☀️ The build tests passed at a506120.

Test	Result	Details
test with	✅	Command did not list any other PRs to include
merge	✅	Merged a506120 at 12a0eab
build (prof)	✅	Log file. Build time: 04 min 17 sec
ceSimReco	✅	Log file.
g4test_03MT	✅	Log file.
transportOnly	✅	Log file.
POT	✅	Log file.
g4study	✅	Log file.
cosmicSimReco	✅	Log file.
cosmicOffSpill	✅	Log file.
ceSteps	✅	Log file.
ceDigi	✅	Log file.
muDauSteps	✅	Log file.
ceMix	✅	Log file.
rootOverlaps	✅	Log file.
g4surfaceCheck	✅	Log file.
FIXME, TODO	✅	TODO (0) FIXME (0) in 0 files
clang-tidy	✅	0 errors 0 warnings

N.B. These results were obtained from a build of this Pull Request at a506120 after being merged into the base branch at 12a0eab.

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