@tree/plot: normalize width.

Add an option to the plot method:

'NormalizeWidth' - a boolean, default is false. If true, then the
tree total width will be adjusted so that it spans over
the 'Width' value. Otherwise, each branch will have a width set by
this 'Width' value.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
importImarisTracks: read header from the first line,

not from the second.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
countIncomingEdges: only use the upper triangular part of the adjacency matrix.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
@scatterpoints/getXY: minor formatting.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
trackMateGetFrom: can now read and return visible spots.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
trackMateGraph: return NaN when an attribute is missing from the XML file.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>