tree variables data: define as per-instance list

[hotzeplotz]

now that we have introduced layers and layer groups, the previous workflow for the loading of tree data files is not making efficient use of network requests.

as tree variables are now defined (as XHR-loaded resources) for each layer, in order to optimize loading of resources for tree data shared across layers, the metadata file should define a master list of XHR-loaded resources and only reference these from individual layers' variables metadata.

we can then XHR-load all the required data files, and use them where necessary.

• update metadata.json schema
  • add a section where XHR-loaded resources are listed
  • reference resources from the master list from within metadata of tree-type variables
• update platform.js code
  • retrieve all tree data files (only for variables X layers that actually contain data)
  • use preloaded tree data files when attaching tree charts to visualization