Implement offsetting work-items when using iterators to SYCL buffers

[tremmelg]

While the SyclIterator interface does provide the position the iterator is pointing to in a buffer, they are not used when retrieving the buffer from the iterators and launching kernels for algorithms.

I've also attached a few tests, two of which fail at the moment. The failing ones have iterators with either the starting or ending position in the buffer altered.
buffer_iterator.cpp.tar.gz

[Ruyk]

can you put those tests on a branch named after the issue? (issue-63) Thanks!

[tremmelg]

Done. https://github.com/KhronosGroup/SyclParallelSTL/blob/issue-63/tests/buffer_iterator.cpp