Improve phaseShifterOptimalTap

[pgambier]

The phaseShifterOptimalTap function be improved :

1. List the use cases
2. Improve the algorithm with AMPL

[nomont]

For one PST, AMPL is useless. Make an implementation based on an hybrid dichotomy-Newton method:

• The initial tap is known.
• The direction to influence the flow in the right direction is known.
• Try a first guess : half of the number of plots between the initial tap and the max (or min) tap
• Compute pseudo-derivative to do an unequal dichotomy where the split is made on the best guess of the solution given the pseudo-derivative.
• Stop at the last tap that guarantees that the flow is below the limit.