Allow for in-place changing of parameters

At the moment BluePyMM assumes every e-model has a fixed set of parameters, and it discards all the morphologies that don't work with this parameter set.
We should allow the e-model package to return a new set of parameters for each morphology, so that it can e.g. run an optimization to fine-tune the parameters.

No files or tests are named. Start by tracing how BluePyMM receives e-model parameters and filters morphologies that do not match the fixed parameter set. The work is done when an e-model package can return morphology-specific parameters and those morphologies are processed without being discarded.

Written by the indexing model from the issue text.