Feature/read time unit

[frasercl]

Problem

website-3d-cell-viewer time series designs show a time step indicator next to the scale bar. To display this we need to know the scale and units of the volume's time dimension.

Solution

• Add fields to ImageInfo: time_scale, time_unit.
• Get OMEZarrLoader to populate them where available.
• Generalize spatialUnitNameToSymbol to handle converting time units to symbols too (as unitNameToSymbol).

[frasercl]

I think including unit-to-symbol conversion with other methods more obviously associated with data loading is still just on the edge of reasonable given the size of the file. But if either category grows by much more, that would probably tip the scale towards separating them out.