load/storeChunk: slice view

[ax3l]

The A.storeChunk(data, offset, extent) and A.loadChunk(data, chunk_offset, chunk_extent) methods should for syntactic sugar provide a slice-view access of the form:

// write (copy)
A[view] = data;

// returns: std::unique_ptr< A.dtype >
auto B = A[view];
// hm, maybe that's not possible, so let's say something like
auto B = A[view].astype< some_dtype >();

[ax3l]

Hm, maybe we can also implements views when we add the xtensor frontend

https://xtensor.readthedocs.io/en/latest/basic_usage.html

related to #32

[ax3l]

Hm, I think we will need this C++ Slice class even earlier if we want to have a seamless python API.
And two helpers for:
Extent, Offset -> Slice and Slice -> Extent, Offset

No need to expose it in the C++ frontend at this point, tough.

[ax3l]

Implemented with #458