UI issues with napari plugin

[vineetbansal]

Some misc. issues we need to take care of with the plugin:

• opening up the plugin before doing anything else results in an IndexError.
• add individual entries for sample data for 4 patients from SCC
• multiple widgets introduced when opening local h5ad files - in no case should the center align / pairwise align widget be added to the dock if it already exists there.
• for center alignment, a progress bar is shown, but blocks interaction with the plugin. This should be running in the background.
• overlap textbox should accept blank values, which then runs paste1 alignment.
• threshold cannot be modified in increments of 1e-3, and thus the default of 0.001 is clamping to 0.00