Support Lazy Loading for Large Signals.

[CSSFrancis]

I was going to go through and make this better for large signals so that you could potentially use this as a way to view and check 4D STEM data. That requires lazy loading to be an option.

This also really would benefit from better saving of navigator signals.

To Do:

• [ ] virtual imaging (non live/ add compute button)
• [ ] better display compute bar to know how long computation should take
• [x] Fix zspy file loading
• [ ] Fix 5D sliders being too small on Mac (test on windows)