Add utility methods to parallelize over blocks

[hanslovsky]

This commits adds

• ParallelizeOverBlocks.parallelize - parallelize a task over blocks and return futures
• ParallelizeOverBlocks.parallelizeAndWait - parallelize a task over blocks and wait for the result
  with various signatures. The three signatures are distingished by the way the blocks for parallelization are specified:

1. Interval interval, int[] blockSize
2. long[] min, long[] max, int[] blockSize
3. List< Interval > blocks

This is a common pattern that I have used multiple times and I think that imglib2-algorithm is the adequate package for such methods.

BlockOffsets.java contains helper methods for generating lists of blocks from intervals and block sizes.

[hanslovsky]

#40 is also relevant for this, in particular for ParallelizeOverBlocks.parallelizeAndWait.

[hanslovsky]

@imglib/admins please comment

[axtimwalde]

Sounds reasonable but in practice I always needed more. We should have a look at this class
https://github.com/saalfeldlab/hot-knife/blob/master/src/main/java/org/janelia/saalfeldlab/hotknife/util/Grid.java
and add what's missing (i.e. cropping the grid intervals to input interval boundaries and related things).

[hanslovsky]

• Rename BlockOffsets to Grids to be closer to imglib2 namespace (CellGrid etc)
• Add tests (detected a bug for min != 0)
• new convenience methods for:
  • retrieving complete intervals (instead of just min)
  • retrieving complete intervals and position within cell grid
    Unfortunately, it is not possible to use CellGrid here because we have to consider a non-zero min, unless we have specific implementations of CreateAndCropBlockToFitInterval and GetGridCoordinates for zero min and non-zero min, respectively. This does not sound reasonable to me, though.

We should still consider moving the Cells class into imglib2 core, though, as discussed in person.

[hanslovsky]

I had a more common use case like this:

void forEachOffset( long[] min, long[] max, int[] step, Consumer< long[] > doAtOffset );

This can be generalized to allow for use with RandomAcess, for example. I will add this before the PR gets merged.

[hanslovsky]

Any comments/objections against a merge? I would like to go ahead and merge this.

[hanslovsky]

@imglib/admins if there are no further objections or comments, I will go ahead and merge this PR.