Modifications to layout_as_stemma

[Jean-Baptiste-Camps]

This function allows to have a stemma where heights of nodes are determined according to number of disagreements and omission/additions towards the model.

For now, it is in absolute value.

Should we modify it to express it as a ratio to the comparable lines (omit lines with NA) ? Because, for now, witnesses with lots of NA are, as an effect, close to their models.