Layer reading is wrong in circuit mvd3 branch

As mentioned by @wizmer, the layer is not read correctly in:
https://github.com/BlueBrain/BluePyMM/blob/read_mvd3_dontmerge/bluepymm/prepare_combos/parse_files.py#L158

It needs to use the layer mapping in the mvd3 file at
circuitmvd3_file['library']['layer']