Add JSON IO for inp::StandaloneInput

[amandalund]

I wanted to add some more documentation for running our standalone apps (in particular creating inputs). Rather than do this for the RunnerInput which we plan to deprecate, I'm going to go ahead and add support for using inp::StandaloneInput with celer-sim (celer-optical already uses inp::OpticalStandaloneInput). This first step adds the remaining json serialization needed.

[github-actions]

Test summary

 5 701 files   9 289 suites   19m 3s ⏱️
 2 326 tests  2 283 ✅  43 💤 0 ❌
32 901 runs  32 730 ✅ 171 💤 0 ❌

Results for commit dedf71c.

♻️ This comment has been updated with latest results.

[sethrj]

Looks good @amandalund ! I think the documentation may need updating.

[codecov]

Codecov Report

❌ Patch coverage is 94.70405% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.32%. Comparing base (72ced17) to head (dedf71c).

Files with missing lines	Patch %	Lines
src/celeritas/inp/EventsIO.json.cc	94.66%	3 Missing and 1 partial ⚠️
src/celeritas/inp/Field.hh	63.63%	0 Missing and 4 partials ⚠️
src/accel/AlongStepFactory.cc	0.00%	2 Missing ⚠️
src/accel/CartMapMagneticField.cc	50.00%	1 Missing ⚠️
src/accel/CylMapMagneticField.cc	50.00%	1 Missing ⚠️
...eritas/alongstep/AlongStepCartMapFieldMscAction.cc	0.00%	1 Missing ⚠️
...leritas/alongstep/AlongStepCylMapFieldMscAction.cc	0.00%	1 Missing ⚠️
src/celeritas/field/RZMapFieldInputIO.json.cc	0.00%	0 Missing and 1 partial ⚠️
src/celeritas/inp/FieldIO.json.cc	98.63%	0 Missing and 1 partial ⚠️
src/celeritas/inp/ImportIO.json.cc	96.15%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2425      +/-   ##
===========================================
+ Coverage    87.22%   87.32%   +0.10%     
===========================================
  Files         1399     1401       +2     
  Lines        44163    44443     +280     
  Branches     13344    13495     +151     
===========================================
+ Hits         38522    38811     +289     
+ Misses        4423     4409      -14     
- Partials      1218     1223       +5     

Files with missing lines	Coverage Δ
src/accel/AlongStepFactory.hh	62.50% <ø> (ø)
src/accel/CartMapMagneticField.hh	50.00% <100.00%> (ø)
src/accel/SetupOptions.cc	74.44% <ø> (ø)
src/accel/detail/MagneticFieldUtils.hh	100.00% <ø> (ø)
...eritas/alongstep/AlongStepCartMapFieldMscAction.hh	0.00% <ø> (ø)
...leritas/alongstep/AlongStepCylMapFieldMscAction.hh	0.00% <ø> (ø)
src/celeritas/field/CartMapFieldParams.covfie.cc	86.95% <ø> (ø)
src/celeritas/field/CartMapFieldParams.hh	0.00% <ø> (ø)
src/celeritas/field/CylMapFieldParams.cc	100.00% <100.00%> (ø)
src/celeritas/field/CylMapFieldParams.hh	66.66% <ø> (ø)
... and 19 more

... and 8 files with indirect coverage changes

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