obervable scatter

[behinger]

this is not ready yet. I asked makie peple on how to do it properly. Right now it doesnt indicate which plot was clicked correctly

• add eegseries observables support
• basic categorical working
• fix label
• first draft of observable for scatter

[vladdez]

New feattures:

• eegseries observables support
• added categorical columns
• support for observable of topoplot markers
• tests for checking new features