bring volume-viewer code into simularium-viewer

[toloudis]

We want to use the volume viewer code in simularium but expect there might be modifications required. We want to be able to track any changes to volumeviewer while concurrent dev happens there. So there are a few options:

1. copy/paste into subdirectory. We estimate this might be bad for change tracking with the main volumeviewer but is very straightforward.
2. git submodule volume-viewer (and use a branch of course)
3. use npm package dependency from a branch of volumeviewer (can use npm link or "file:" dependency for local dev).

Any of these may require changing exports. Goal should be to do minimal changes.
Consider the following as a nice goal: we will need to be able to import the Volume class, and the IVolumeLoader infrastructure, directly. We can also probably use the VolumeDrawable and RayMarchVolume for rendering directly inside of simularium (with no modifications?)

This task is complete when we have a strategy where we can import any volume-viewer modules into simularium-viewer.

[frasercl]

Discussed with @toloudis and we agreed that adding volume-viewer the traditional way (as an npm package) may give us everything we need with minimal overall changes to the API surface. Our plan for development of this feature is:

• Create a branch on volume-viewer on which to make these API changes.
• During development of this feature, install volume-viewer as a local package, i.e. with a dependencies entry of the form "@aics/volume-viewer": "file:../volume-viewer",. Assume that developers and reviewers have volume-viewer stored at this location and on the correct branch.
• Once initial implementation of volume agents is complete, PR the branch on volume-viewer. Assess whether the changes are superficial enough to merge and release, or if we need to take a more unusual approach to integrating the code into simularium.