align openmc plots on-the-pixel

[moellep]

The openmc mesh is all within the mesh dimensions, rather than having the boundaries on the half-pixel. In the UI, this value needs to be set:
SIREPO.PLOTTING_HEATPLOT_FULL_PIXEL = true;

and on the 2d geometry slices, the values will need to be adjusted centerward, and should match the openmc-plotter.

It looks like the 3d plots are already plotting the correct way.

Corrected 2D:

3D: