[ENHANCEMENT] 3D Viewer atom list should be a tree

[MBartkowiakSTFC]

Is your feature request related to a problem? Please describe. At the moment it is only possible to change the atom size and colour for a single atom.

Describe the solution you'd like The atoms should be grouped in a tree structure, allowing the parameters to be changed for an entire group, while at the same time still allowing individual atoms to be customised.

Describe alternatives you've considered There is the open question of how the atoms will be grouped: by element, by molecule, etc.

Additional context N/A

[MBartkowiakSTFC]

For performance reasons, this tree should also be populated the lazy way. Only if somebody tries to unfold the list of individual atoms of a given type, then we should add the items there. Otherwise it may be too slow for trajectories with millions of atoms.