Prompt user to load local geometry files

[blairlyons]

• [ ] https://github.com/simularium/simularium-planning/issues/121
• [ ] https://github.com/simularium/simularium-planning/issues/117

Use case

As a user with a local .simularium file that includes local paths to geometry files, I need to know how to include the geometry files when loading my data in the Viewer.

Acceptance Criteria

• it is clear to a user that has local geometry files how to drag and drop them
• this is a discoverable feature