ParallelFor with BoxND (#4052)

## Summary

This PR adds the ability to use ParallelFor(RNG) and Loop(OnCpu)
variants with BoxND.

## Additional background

The functions supplied to ParallelFor etc. can have one of the following
input parameter combinations when used with `BoxND<dim>`:
``` C++
(int, int, ...) // dim times
(IntVectND<dim>)
(int, int, int) // if dim is 1 or 2, for backwards compatability
```
After that come additional parameters such as `int n`, `Gpu::KernelInfo`
or `RandomEngine`.

I had to mark the constructor of GPU Handler as explicit to avoid
ambiguity between (i, j, 0) and (i, j, Gpu::Handler).