Add applySystemOperator(result, x) to Residual class for PCG support

[julianlitz]

This PR introduces the applySystemOperator(result, x) method to the Residual class, enabling computation of the system operator applied to a vector (A * x). Previously, the class only supported computing the residual, but PCG also requires direct application of the system operator.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.18%. Comparing base (39ce1dd) to head (4551577).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #186   +/-   ##
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  Coverage   95.18%   95.18%           
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  Files          94       94           
  Lines        9345     9355   +10     
=======================================
+ Hits         8895     8905   +10     
  Misses        450      450           

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[EmilyBourne]

I don't understand what is done in this PR. The description talks about new calculations but the existing calculations seem to have been changed in a non-trivial way (e.g. += -> -=)

[julianlitz]

@EmilyBourne

In ResidualGive what we did before was

RESIDUAL
result = f
result -= A *x

Now we do:

APPLYA
result = 0
result += A*x

RESIDUAL
result = ApplyA(x)
result = f - result

Thats why the sign changed.