[FEATURE] Add support for Three Winding Transformers

[Thijss]

Describe the feature request

It would be nice to include support network configurations with so-called three winding transformers.

In my understanding these transformers can connect three nodes with different voltage levels.

In the current data model, a Transformer can only connect two nodes: (from_node) and (to_node)

current:

[from_node]--[transformer]--[to_node]

requested support for:

[node_1]--[transformer]--[node_2]
                     |
                     |--[node_3]

Acceptance criteria

• Implemented ThreeWindingTransformer
• Unit tests for ThreeWindingTransformer
• Validated with pandapower for symmetric calculations
• Added validation of values to validate_input_data and validate_batch_data
• Documentation updated