[get|set]Data support for selection in addition to count+offset

In nixpy, if one tried to slice by a list of points, i.e. `DataArray.data[[1,5,9,10,23], :]` we currently have no clean way to transfer this into a call to `getData` data that would give us the desired result. HDF5 has a selection based API and we are internally already using it most of the time. I propose we add `[get|set]Data` overloads that take a new selection object.
The only question that immediately comes up is whether we want a native selection object or a front-facing one that facades the HDF5 one.