Structural Metadata: answer todos

[gkjohnson]

Open Questions

• Are attribute accessors usable with non point objects?

TODO

• Should defaults be null? Instead we should get a reasonable initialization of the type? Zeros?
• Final test, code review
• ~Ensure matrix storage order is the same as three.js matrix elements~
• ~Ensure values are normalized before scaling and offsetting~
• ~Handle "no data" correctly such that it returns null instead of a default value?~ (Docs here state it should use the default)
• ~Adjust getPropertyTableData function to optionally take non-array values~
• ~Expose used texture from mesh features info and property textures~
• ~Upload index textures to data set~
• ~Add documentation to PLUGINS.md (meta data plugins, texture overlay, etc)~