remove Pipe reference from mobject::Placement

[ichthyo]

In the first draft, Placement was meant to contain some fundamental coordinates, plus an open ended collection of additional coordinates. This is mostly obsolete by now and should be handled rather by a placement solution. (see #100)

Especially, storing a direct track number has been changed into a Pipe reference -- which meanwhile is considered a bad idea, because it interferes with proper handling of multichannel media. Now I'd prefer to rely on the placement scope concept solely, and derive the other informations on demand.