Fix data access on device

[tpadioleau]

This error appeared when running tests with CUDA for spack/spack-packages#5820

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.50%. Comparing base (8e52bf2) to head (4df75ec).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #312   +/-   ##
=======================================
  Coverage   97.49%   97.50%           
=======================================
  Files          79       79           
  Lines        8629     8641   +12     
=======================================
+ Hits         8413     8425   +12     
  Misses        216      216           

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[julianlitz]

You could use
levels_[0].rhs() and levels_[0].residual() as temporary storage to avoid this vector allocation.

Note that rhs() is filled with correct values at the start of each gmgpolar.solve() call. Also rhs() is no longer used after this writeSolutionToVTK function.

[julianlitz]

Suggested change

	Vector<double> Fx("Fx", grid.numberOfNodes());
	const int level_depth = 0;
	Vector<double> Fx = levels_[level_depth].rhs();

[julianlitz]

Suggested change

	Vector<double> Fy("Fy", grid.numberOfNodes());
	Vector<double> Fy = levels_[level_depth].residual();