Heuristics for generating 'idiomatic' LEMS

[borismarin]

Currently, the LEMS backend will generate correct NeuroML for a limited number of mod files, via building new ComponentTypes for each ionic mechanism found. Ideally, HH-like currents (kinetics, rate variables...) should be mapped to builtin NeuroML types, instead of creating new ones. That will involve clever symbolic manipulation (trying to cast dynamics/current laws into preexisting forms), but should be feasible for simple mechanisms.