accessing single or sliced from the repository

[mikibonacci]

In some cases we might have a large amount of data stored in the repository even for a single structure (large number of sites, properties), so we want to be able to access properties in smart ways:

• only one specific property
• properties in a sliced way

[mikibonacci]

This is partially fixed in #88, where we introduce an additional parameter to the _load_properties_from_npz in StructureData to allow accessing only single properties from the repository npz file. In this way, we don't need to load the whole compressed object.

In the same PR we also introduce slicing of structures, which returns a new object with the selected number of sites.
This is not exactly what we want in this issue, as in this issue we want to only read sliced chunks of the compressed npz file.