Add support for adding a Phyx file to an existing Phyx file

[gaurav]

Problem statement: A user has two Phyx files that they want to simultaneously visualize: either they wish to build a single ontology containing phyloreferences from both files, or they wish to resolve phyloreferences from one Phyx file against phylogenies from another file.

User interface implementation:

1. Add a "Add Phyx file to current file" button.
2. On clicking this button, we ask the user to choose a Phyx file to add.
3. All phyloreferences and phylogenies from the second Phyx file are added to the in-memory Phyx file.

Merging two Phyx files in this way should be straightforward but should be implemented in phyx.js.

[gaurav]

• Corroborate assessment that this is really easy -- if so, then we should do this.
  • Actually, we might want to account for the case where someone tries to import a Phyx file with the same local ID duplicated.
• What if someone tries to add the same file twice in a row?
  • Possible solution: if you see a duplicate ID, skip just the duplicate identifiers.
  • A "Cancel import" button?
• Confirm that combining ontologies from nQuads is already doable purely with the command line (if so, we need this to be documented somewhere).
• Double-check exactly what the paper says about this (and maybe link to the documentation).