Enable synonyms for scientific terminology

[patrick-austin]

Lucene supports synonym files, which can be used for both alternate spellings (e.g. "ionisation" vs "ionization") and scientific terms which are equivalent (such as chemical symbols to element names).

This can either be applied on indexing, on search, or both. It seems that injecting synonyms at search times is the expected approach (https://lucene.apache.org/core/8_5_1/core/org/apache/lucene/analysis/package-summary.html?is-external=true).

The Solr format for synonym files is supported by both elasticsearch, so should be compatible if we move to that.

The other thing to consider is where in the current analyzer we put the synonym injection. To recognise chemical symbols, keeping upper case letters might be useful? However this might be problematic for going the other way round, e.g. hydrogen, Hydrogen and HYDROGEN are all equally valid.