Fixed interpolation rounding for BC4

[RunDevelopment]

Fixes #12

Hi again! Although the discussion around #12 seems to have died down a bit, I decided to make a little PR to fix this issue by fixing the rounding.

Why is the old implementation incorrect?

Integer division truncates the result. For non-negative numbers, this is equivalent to rounding towards zero (floor). To achieve rounded integer division for a divided by b, the old trick is (a + b/2) / b (where / is the truncating division C uses).

This works because for any $a \in N$ and $b \in N, b >0$, the following is true:

$$ round(a/b) = \lfloor a/b+0.5 \rfloor = \lfloor (a + b/2) / b \rfloor = \lfloor (a + \lfloor b/2 \rfloor) / b \rfloor $$

(The last equality is true, because $\lfloor (a + \epsilon) / b \rfloor = \lfloor a / b \rfloor$ for any $\epsilon \in R, 0 \le \epsilon < 1$.)

So if we want to have rounded division by 5 and 7, we need to add 2 and 3 respectively.

The current implementation of (a + 1) / 7 is equivalent to $\lfloor a/7+0.1428572 \rfloor$ or $round(a/7 -0.3571428)$. This makes absolutely no sense mathematically. I think what happened here is that the author knew the trick for rounded division by 2 and 3 ((a + 1) / 2 and (a + 1) / 3 respectively) and incorrectly generalized it to other dividends.

What is the correct behavior?

I strongly believe that rounded division is the correct behavior for the reasons I explained in #15 for BC1. To summarize: I think that the result should be the same as if we were doing everything with floating-point numbers and then converting to integers (via rounding) at the very end. With my suggested fix from #15 being merged already, this new behavior for BC4 (and all that use it internally) would be consistent with BC1.

What did I change?

I changed the + 1 to the correct constants for rounded division by 5 and 7 to fix the rounding. This should come at no additional performance cost, although I haven't benchmarked it.

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This comment from #12 suggested using floor/truncated division. I believe that this behavior is incorrect, but I'm in no way opposed to enabling this behavior via a compile-time option, allowing users to choose. (Although I think rounded division should be the default.) The goal of this PR is simply to fix the current incorrectly implemented rounded division.