Support "Cell Body" for ASC files

[asanin-epfl]

successor of https://github.com/BlueBrain/NeuroM/issues/880.

Interpret "Cell Body" entries as soma. It's an open question how to do that. Need advice from scientists.
According to @lidakanari: a single "CellBody" is considered to be a contour.

For now lets consider multiple "Cell Body" as an error. No one from scientists clarified how to interpret the multiple case.

[asanin-epfl]

Closing in favor of the moved from NeuroM #300.