PERF: dispatch on UnitSystem value in _get_multiplier

[GabrielKS]

Before:

After:

On the current @scoped_enum implementation, UnitSystem.NATURAL_UNITS isn't a constant, it's a dictionary lookup — one that incurs 80 bytes of memory allocation. Thus you can see the performance impact piling up in the "before" case in the order in which the unit systems are checked — DEVICE_BASE, then SYSTEM_BASE, then NATURAL_UNITS. There are a few ways one might fix this:

1. Alter @scoped_enum to make UnitSystem.NATURAL_UNITS behave more like an immutable struct lookup, either by actually making it one or doing something to clue the compiler into the fact that it will never change. This would be the best, most holistic solution, but I couldn't immediately figure out how we might do it without completely overhauling @scoped_enum. I'll keep it in the back of my head though.
2. Instead of testing setting.unit_system == IS.UnitSystem.NATURAL_UNITS, do setting.unit_system.value == 2, etc. No dictionary lookup, but too hardcoded for me.
3. The multiple dispatch approach given here.

Most of the cost savings are from dispatching on the UnitSystem value. Adding another layer of dispatch on the nothing-ness of get_internal(c).units_info seemed to make it a bit faster on top of that, so I've done that too.

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 84.35%. Comparing base (a603b75) to head (91bc0ab).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/models/components.jl	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1216      +/-   ##
==========================================
- Coverage   84.37%   84.35%   -0.02%     
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  Files         181      181              
  Lines        8382     8375       -7     
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- Hits         7072     7065       -7     
  Misses       1310     1310              

Flag	Coverage Δ
unittests	84.35% <42.85%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
src/models/components.jl	61.53% <42.85%> (-4.57%)	⬇️

[daniel-thom]

Do we need this one or can let Julia do its normal behavior?

[GabrielKS]

I'm good with letting Julia do its normal behavior on this one.