New endpoints like /tap/capabilites/MolecularChemicalSpecies

[ivh]

From an email by C.Hill:

maybe we could extend the capabilities-function in order to return all distinct values within the database for a given restrictable/keyword. If the database provides the restrictable "MolecularChemicalSpecies" then
the query:

/tap/capabilities/MolecularChemicalSpecies

should return the list of distinct values from the database.

I do not know if this or something similar is feasible (return the values via returnables), but I should be easy include it in the django interface and it would also solve the problem of dictionaries being up-to-date and we could easily find out if different values are used for the same thing.