Multiscene: parse ro-crate

[frasercl]

Use Case

We need an input format to describe multiscene data.

Solution

ro-crate is the standard way to describe a multiscene OME-Zarr image. Implement basic parsing of ro-crate to get a list of scenes. Use this to drive multiscene loading as described in #275.

[frasercl]

consider adding a control to the testbed viewer. should already have test data to apply to it from #275

[toloudis]

Consider adding a different ticket to load a format that can be exported from BFF