[WIP] Optional brute force approach for RDF calculation

[padix-key]

This PR aims to improve the performance of biotite.structure.rdf() (issue #307): Although a CellList scales well for a large number of centers, the brute-force approach, i.e. the pairwise distance measurement of all atoms is faster for a small number of centers. This PR adds an optional use_cell_list parameter to give the user control about the approach. By default, the appropriate approach is chosen based on a threshold number of centers.