[WIP] Storing and re-using atom selections

[MBartkowiakSTFC]

Description of work This PR, once complete, will implement a way of storing, previewing and re-using atom selections. The atom selection definitions are meant to be abstract and do not have to be tied to a specific trajectory file.

Fixes Complete:

1. Added the option of loading a selection from an MDA file. In progress:
2. Planning to add an extra tab to the GUI, which will allow users to choose one of the stored selection operation sequences, and preview the effect it will have on a specific trajectory.
3. Planning to add an extra selection operation, which will group and apply atom selection done in the 3D view by clicking on individual atoms.
4. Initial work has been done on adding code to the backend for storing atom selections.

To test Will be filled out later.