Improve mc_network label display

[NataliaCiria]

• Percentages: By default, all stochastic nodes are considered probabilistic and represented with a "%", even if they have an "n_" prefix. Whether they're probabilistic or not could be defined in mctable or assumed with the existing logic, but never if they exceed 100%. There should be an option to disallow percentages.
• Variates: By default, the first variate is used. This makes sense because totals will have fewer variates than data-created nodes, and if multiple data frames are used for different nodes, filtering becomes tricky. However, it should be possible to select which variate is used or to filter with certain conditions.