Speed up bar states bounds calculation of entropies

[bennibolm]

This PR speeds up the calculation of bounds for mathematical and specific entropy.
Before, we looped through every node, look at the bar states on the right, top, left, bottom and calculated the entropy every time. This resulted in doubled entropy calculation.

Now, we just look at the bar states on the left and bottom, calculate the entropy and save it for the node itself and the respective neighbor. In the end, we add the calculation at the interface/boundary far right and top.

The new implementation only takes about 70% of the time the old one took.
Before:
calc_variable_bounds! 4.34k 992ms 20.4% 229μs 577KiB 19.9% 136B
Now:
calc_variable_bounds! 4.34k 704ms 15.5% 162μs 577KiB 19.9% 136B

This is not necessary for the calculation of the bounds for the local limiting, since there we can directly use the bar states without further calculations.

[github-actions]

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[bennibolm]

I think this PR is directly mergeable. Of course, I checked that we get the same results as before and (hopefully) all tests pass.
Could you take a quick look anyway?@amrueda

[amrueda]

Looks great. Just one comment: