Split IterableRegion

[tpietzsch]

Currently, IterableRegion implements both RandomAccessibleInterval<BooleanType> and IterableInterval<Void>.

The idea is that if you have a RandomAccess, it will be true inside the region, and false outside. When you iterate you only want to visit the inside pixels (where the RA would be true). Therefore IterableInterval<Void>, not BooleanType.

It seemed at one point neat (albeit confusing) to implement this by inheriting Accessibles of different Type.
With imglib/imglib2#312 this will no longer be possible.
IterableRegion.getType() cannot return both Void and BooleanType.

Therefore, the "inside IterableInterval" should become a separate thing. maybe by adding a method IterableInterval<Void> inside().