Add network constraints for load flow analysis

[alihamdan]

The constraints are not used by the load flow solver. They are only used to analyze the results.

The constraints that can be defined include:

• For buses: minimum and maximum voltages. Use bus.res_violated to see if the bus has over- or under-voltage.
• For lines: a maximum current. Use line.res_violated to see if the loading of any of the line's cables is too high.
• For transformers: a maximum power. Use transformer.res_violated to see if the transformer loading is too high.
• The new fields also appear in the data frames of the network.

I still need to change the documentation.

[benoit9126]

👍 The documentation remains and it will be good!