Multiterminal HVDC

[JosepFanals]

Is your feature request related to a problem? Please describe.
We need to model VSCs with three terminals (2 on the DC side, 1 AC) to be able to represent various HVDC schemes.

Describe the solution you'd like
The VSC should be a 3 terminal component. There should be the capability to set DC buses to the ground. The control capabilities should stay the same. Internally, we have to solve the current balance, not the power balance, on the DC side.

Describe alternatives you've considered
Using the current version, yet we cannot symbolize a metallic return.

Additional context
Check Jordi's doc

Implementation of a bipolar DC system in GridCal_v3.pdf