Fix some array sizes in the Maxwell code

Micael Oliveirarequested to merge
fix_out_of_bounds_maxwell into develop

Description

Change the size of several arrays in the Maxwell boundaries from MAX_DIM to 3, as the code assumes in many places that MAX_DIM=3. Although this had no effect on the results, in some places this was causing floating-point exceptions if the code was compiled with --max-dim=4 and with the -finit-real=snan -ffpe-trap=invalid compiler flags.

News snippet

Fix some array sizes in the Maxwell code

Checklist

  • I have checked that my code follows the Octopus coding standards
  • I have added tests for all the new features added in this request.

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