MapNeighborhood Concurrency Issue

Code to generate the problem:

```
@Test
public void test() {
ops.run(MapNeighborhood.class, out, in,
new AbstractUnaryComputerOp, ByteType>()
{

@Override
public void compute1(Iterable input, ByteType output) {
for (ByteType e : input)
e.copy();
}
}, new RectangleShape(1, false));
}
```

where `out` and `in` are two `Img`.

The test above basically iterate through all the 3x3 rectangular neighborhoods in the input image, and do a `copy()` on each pixel of each neighborhood. This test always `ArrayIndexOutOfBoundException` at `ByteArray#getValue(index)`.

Other observations:
1. changing to `FloatType` does not help
2. `DiamondShape`, `HorizontalLineShape`, still produce the problem, but `PairOfPointShape` does not
3. When the shape is changed to `new RectangleShape(0, false)` (1x1 rectangle so no overlap between nearby shapes), this test does not throw exception.
4. change `copy()` to some method call that does not access the value (e.g. `getEntitiesPerPixel()`) does not produce the problem
5. [`MapNeighborhood`](https://github.com/imagej/imagej-ops/blob/master/src/main/java/net/imagej/ops/map/neighborhood/MapNeighborhood.java#L81) uses `shape.neighborhoodSafe(...)`, which seems to be "thread safe" to me (I do not know how those `shape`s are implemented).

I think the reason of this behavior is that each pixel is accessed concurrently, in different `RectangleShape`s. I suspect that those shapes are not implemented correctly.

Since any useful use of `MapNeighborhood` need to access the pixel values in the neighborhood, I think this issue is quite serious. Or do I misunderstand how this op should be used? @dietzc @ctrueden

Start with the reproduction in the issue and MapNeighborhood.java around the neighborhoodSafe call. Inspect the relevant shape implementations, especially RectangleShape and the overlapping-neighborhood path, while comparing against PairOfPointShape and a 1x1 RectangleShape. Done means the reproduction can read neighborhood pixel values concurrently without ArrayIndexOutOfBoundsException, with a regression test covering the failure.

Written by the indexing model from the issue text.