String selections for non-MD molecules

[BradyAJohnston]

Via MDAnalysis string selections are now possible, using the MDAnalysis selection language.

Possibility to also port this to Molecules. Could be done by turning the molecule into a Universe on Import, and using that for creating the selections. Could also potentially be done via using some of the lower level selection parsing of MDAnalysis, and then turning that into our own custom boolean creation via biotite.

There will likely be support for biotite selection strings in the future, but then there would be multiple selection languages in a single interface which isn't good. Best to try and keep it limited to a single language.