Dump emodel HOC to intermediate format

At the moment the HOC files are generated directly by bluepyopt from the models in memory.
To support other backends and analysis, the models should be dumped to an intermediate format.

Start by tracing how bluepyopt currently generates HOC files from the in-memory models. Define the intermediate representation needed to support other backends and analysis, then verify that model dumping produces a usable representation instead of only direct HOC output.

Written by the indexing model from the issue text.