Implement implicit derivation of mechanisms. (#817)

Implement implicit derivation of mechanisms.

* When a mechanism of the form "foo/param=value,..." or "foo/newion" is
requested from a mechanism catalogue, attempt to perform a derive the
information or override set from the mechanism "foo" using the given
global parameter settings and/or ion remappings given in the name.
* Split mechanism catalogue implementation into a private catalogue
state with non-throwing behaviour and without any implicit mutation,
with mechanism_catalogue methods responsible for throwing any generated
errors, with the aim of avoiding throw/catch in expected program flow.

Implements #807.