Added voltage stability indicators for branches: Fast Voltage Stability Index (FVSI).

[tank723]

Fast Voltage Stability Index (FVSI) for Voltage Stability Analysis in Power Transmission System. https://doi.org/10.1109/SCORED.2002.1033108

Added functions in vs_matrix:

• getBranchVoltages : Returns voltage sending, receiving
• getImpedanceMagnitude : Returns Z = (1 / (B2 + G2))
• getVoltageStabilityIndex : Returns FVSI = 4(Z2 * Qij/Mbase)/(Vi2 * X)

Modified vs_matrix serialwrite function to include FVSI calculations to the branch output.