remove dependency to neurondb.xml

This file is a BBP internal format. It may also contains informations that are unrelated to bluepymm such as repair instructions.
We should have a much simpler format for the end user.

Start by locating where bluepymm depends on neurondb.xml and how that format is parsed or generated. Determine the requirements for a simpler end-user format that excludes unrelated repair instructions; the work is done when bluepymm no longer depends on the BBP internal format and the replacement behavior is covered.

Written by the indexing model from the issue text.