Clarify asymmetry of get_all_unitary_topologies_set and get_all_unitary_topologies_change.

[MatthijsdeJ]

The functions get_all_unitary_topologies_set and get_all_unitary_topologies_change do not return symmetric (w.r.t busbar) actions. This behavior is not documented. It would be good to clarify this.

I suggest the modification:

To be valid a topology must satisfy:

        - there are at least one side of the powerline connected to each busbar (there cannot be a load alone
          on a bus or a generator alone on a bus for example)
        - the topology is not a symmetric copy (w.r.t. one busbar) to an already determined topology
        - if `add_alone_line=False` (not the default) then there must be at least two elements in a 
          substation

[BDonnot]

Hello,

You are totally correct. The mentioned methods return the list of unique possible topology for each type of actions, considering all elements are connected.

For example, if a substation counts 4 elements, then the topologies:

• (1,1,2,2)
• (2,2,1,1)

Will give the exact same output.

Indeed in both cases the elements 1 and 2 are directly linked together, and the element 3 and 4 are directly linked together. But (1,2) and (3,4) are not directly linked together.

I'll try to be more clear on the description of these methods.

I hope it clarify the behavior.