Handling missing nodes and edges in flatmap

[napakalas]

This PR:

• Using connectivity_terms mechanism to deal with aliasing of missing nodes.
• Add a script, npo_check.py, to identify missing nodes and edges (in tools).
• Add a script, npo_align.py, to align the missing nodes to candidate terms (in tools)
  • Add an optional dependency group, alignments, in poetry.toml, with two dependencies
• Add a script, npo_alias.py, to merge the curated aligned missing nodes with connectivity_terms.json