Remove external variable copy for PowerElectronics

[alexander-novo]

Description

Removes PowerElectronicsModel::distributeVectors() by giving components access to pointers directly to external variable in the system vector.

Speedup (develop v.s. this PR):

1000 IBRs

Benchmark 1: InstallBench/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 500
  Time (mean ± σ):      2.471 s ±  0.012 s    [User: 2.451 s, System: 0.018 s]
  Range (min … max):    2.456 s …  2.498 s    10 runs
 
Benchmark 2: Install/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 500
  Time (mean ± σ):      1.339 s ±  0.012 s    [User: 1.320 s, System: 0.018 s]
  Range (min … max):    1.322 s …  1.355 s    10 runs
 
Summary
  Install/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 500 ran
    1.84 ± 0.02 times faster than InstallBench/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 500

4000 IBRs

Benchmark 1: InstallBench/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 2000
  Time (mean ± σ):     21.007 s ±  0.209 s    [User: 20.933 s, System: 0.064 s]
  Range (min … max):   20.590 s … 21.306 s    10 runs
 
Benchmark 2: Install/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 2000
  Time (mean ± σ):      6.417 s ±  0.052 s    [User: 6.357 s, System: 0.056 s]
  Range (min … max):    6.323 s …  6.524 s    10 runs
 
Summary
  Install/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 2000 ran
    3.27 ± 0.04 times faster than InstallBench/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 2000

10,000 IBRs

Benchmark 1: InstallBench/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 5000
  Time (mean ± σ):     74.876 s ±  0.181 s    [User: 74.715 s, System: 0.129 s]
  Range (min … max):   74.653 s … 75.276 s    10 runs
 
Benchmark 2: Install/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 5000
  Time (mean ± σ):     23.310 s ±  0.182 s    [User: 23.174 s, System: 0.124 s]
  Range (min … max):   23.060 s … 23.687 s    10 runs
 
Summary
  Install/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 5000 ran
    3.21 ± 0.03 times faster than InstallBench/examples/PowerElectronics/ScaleMicrogrid/scalemicrogridarbitrary 5000

Proposed changes

Each component keeps track of y_ext_, yp_ext_, and f_ext_, which are arrays of pointers to the values of external variables, their derivatives, and their residuals.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments