We want to be able to add normalization for additional units without needing a full reload.
Note that we don't need to be able to change an existing conversion; that would be nice to have, bit it's not a requirement.
Possible implementation strategy, option I:
* Find all values referencing the respective unit using a SPARQL query
* Find the statements using that value, and the items using that statement
...option II:
* Find all values referencing the respective unit using a SPARQL query
* Find the statements using that value
* Compute the normalized value using the new mapping, and add them to the triple store.
...option II|:
* Find all statement values referencing the respective unit while scanning a JSON dump
* Compute the normalized value using the new mapping, and output them (as n-triples or turtle).
* Bulk-load the new triples into the query service