[ENHANCEMENT] Change appearance of a single atom in 3D view

[MBartkowiakSTFC]

Is your feature request related to a problem? Please describe. At the moment, the 3D view can only change the size and colour of all the atoms of one type. Our users would like an option of marking a single atom to find it easily in different frames.

Describe the solution you'd like

1. Create an option to add an extra row to the atom types. This could contain atoms of any type, and should override the default settings for that atom type.
2. Combine this with the atom selection helper, so multiple atoms can be added to the new row.

Describe alternatives you've considered N/A

Additional context N/A