Option to deselect all areas and default of None

[brianhhu]

I couldn't find how to deselect all structures using the CCFwidget- it seems at least one structure has to be selected. Currently, selected_acronyms and selected_allen_ids are initialized to None, which (unintuitively) actually selects all structures. It might be helpful to have something more like an 'all' option for all structures, a 'None' option for no structures (i.e. label map weights set close to zero), and the ability to select certain structures when passing either selected_acronyms or selected_allen_ids.

Relatedly, re-setting selected_allen_ids or selected_acronyms back to the default of None (e.g. using cc.selected_acronyms = None) results in an error.

[thewtex]

Please take a look at #22