summed-area 1.0.0

Implementation of a summed-area table for fast sums or averages of subsections of a 2d array or an image
Documentation 
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# Summed Area Table AKA Integral Image

It precomputes sums of all rows and columns in a 2d array for fast `O(1)` querying of sums of areas within it.

It does this:

```rust,ignore
let mut sum = 0;
for row in y1..y2 {
    for col in x1..x2 {
        sum += input[col + row * width];
    }
}
```

but faster:

```rust,ignore
// precompute
let s = SummedArea::new(input, width);

// now it's fast:
let sum = s.sum_range(x1..x2, y1..y2);
```