Jmm/remove implicit logic

[Yurlungur]

PR Summary

Resolves the bug found by @clellsolomon and reported in #423. In short, the issue is that a package can have a different label in StateDescriptor than key in Packages_t, however they were assumed to be identical in the code I wrote in MR #386. This MR undoes that assumption by explicitly using keys instead of package labels. I will submit an MR in the future that resolves the ambiguity where each package has two "names."

PR Checklist

• Code passes cpplint
• New features are documented. (None)
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md

[codecov]

Codecov Report

Merging #424 (6fee1ad) into develop (4c48a10) will increase coverage by 0.05%.
The diff coverage is 77.14%.

@@             Coverage Diff             @@
##           develop     #424      +/-   ##
===========================================
+ Coverage    56.52%   56.58%   +0.05%     
===========================================
  Files          108      108              
  Lines        11055    11078      +23     
===========================================
+ Hits          6249     6268      +19     
- Misses        4806     4810       +4     

Impacted Files	Coverage Δ
src/driver/multistage.hpp	100.00% <ø> (ø)
src/interface/meshblock_data.hpp	60.00% <ø> (ø)
src/interface/swarm_container.cpp	0.00% <0.00%> (ø)
src/interface/swarm_container.hpp	18.18% <ø> (ø)
src/interface/variable.hpp	48.38% <0.00%> (ø)
src/mesh/mesh.hpp	44.68% <ø> (ø)
src/outputs/restart.hpp	86.84% <ø> (ø)
src/parameter_input.hpp	100.00% <ø> (ø)
src/mesh/mesh.cpp	52.05% <50.00%> (-0.04%)	⬇️
src/outputs/outputs.hpp	50.00% <66.66%> (ø)
... and 9 more

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[Yurlungur]

On @AndrewGaspar 's suggestion, I will close this and simply merge in #425 as appropriate.