Fix Raster Initialization

[ilan-gold]

Background

Currently defining only a partial channels object or some other partial aspect of spatialRasterLayers does not really work because channels, for example, is not a coordination type. So this PR merges the initial spatialRasterLayers with the auto-generated one and then forces the hook to set the value

Change List

• Allow for partially defined spatialRasterLayers coordination values

Checklist

• Ensure PR works with all demos on the vitessce.io homepage
• Open (draft) PR's into vitessce-python and vitessce-r if this is a release PR