[IMPROVEMENT] Add source node when Sn node is parsed in Grid.from_txt

[wwadman]

Describe the problem

Suppose we create a grid from text with a source node like this:

grid = Grid.from_txt(
    "S1 2",
    "S1 3 link",
    "3 4 transformer"
)

Then source node S1 does not show up in grid.source, which is confusing.

Improvement proposal

I guess the procedure under https://power-grid-model-ds.readthedocs.io/en/stable/examples/model/grid_examples.html#adding-a-source should be when an "Sn" node is found in the Grid.from_txt method?

[vincentkoppen]

The S in S1 does not stand for source, but substation.
So all it does it set the node_type of node with id 1 to NodeType.SUBSTATION.

A source has all kind of different properties, we are working on better serialization support in #100. Once that is merged, a grid can be exported/imported from a json. Which can include the source.

[vincentkoppen]

#100 has been merged