Add channel-width to box-plot titles
[NB] Add functions to compute wire-length cost...

[NB] Add Cython-exposed functions to compute wire-length cost from:

 - Net-keys
 - Block-keys
 - Net-connection-counts _(i.e., number of blocks connected to each net)_.
 - Block-positions

Both _bounding-box_ and [_Star+_][1] costs are available.

[1]: http://dx.doi.org/10.1016/j.vlsi.2011.02.001