vol*: multiple volume viewer (non simultaneous)

[alisterburt]

This is a high value feature for volume visualisation identified in #1086

We often need to annotate large sets of volumetric data and want to navigate between different volumes quickly from the UI.

• The volumes do not share a coordinate system, they are independent from each other.
• The resulting annotations are aggregated and analysed in bulk.
• only one volume should be stored in memory per time, potentially with some caching to avoid potentially expensive reloading