Implement mechanism ion rebinding. (#786)

* Make global parameters and ion rebindings part of the instantiate interface, rather than insist that all concrete mechanisms implement these as methods.
* Mechanism catalogue instance() returns a pair, comprising the concrete mechanism for the requested backend, together with the override data.
* Extend catalgoue derive() method to take a list of old ion name -> new ion name remappings for a mechanism.
* Add exceptions for ion remapping errors, and check for these errors.
* Add convenience function for reparameterizing a mechanism with a single ion dependency over other ions. (This will be used for the future nernst pre-supplied mechanism.)
* Add unit tests for: chained renamings of ion names across multiple derivations; correct shared state ion assignment after renamings; ion remapping exceptions; parameterize_over_ion.