Handle unquoted labels better

In some cases, the value for a field needs to be an ontology term. Since the value could be an arbitrary class expression, the simplest form for a name with spaces in it requires extra quoting: `"'biological process'"`. We should see if this can be made less confusing for authors.

The issue names no files, tests, or entry points. Start by locating how field values and ontology terms are parsed, then clarify the intended syntax for names containing spaces. Done means the accepted authoring form is unambiguous and covered by tests.

Written by the indexing model from the issue text.