ENH: Compute voltage variations on lines

[alihamdan]

Something like line.res_voltage_variations and a voltage variation column in the results dataframe of lines to calculate the voltage drop/rise for lines.

Not sure if we can do it for transformers in all cases; maybe for three-phase transformers and single-phase transformers but not center-tapped?

[benoit9126]

For transformers, we could compute res_voltage_variation_levels if nominal voltages have been given for the HV and the LV buses. The same could also be computed for lines.