build PHLTIModel from_interconncetion - #2648

This PR introduces a new classmethod for PHLTIModel called from_interconnection. The idea is that the method takes a sequence of PHLTIModel (currently S, N and P have to be zero, this can be generalized but I first have to work out the math) and connects them via lossless gyrator interconnection.

Technical description of how the interconnection should be passed

In order to specify which models are connected with each other, a sequence of connections (just a list / tuple) is passed. An entry in connections is a tuple / list of three elements. The first two entries are tuples / lists themself of length two and contain the model index as first entry and as second entry and another tuple / list of lenght 2, specifying the space of the input matrix G of the model (blocks if G has a BlockVectorSpace or idx if not) and which of the blocksubspaces / indices of G should be interconnected. The third entry in top-level tuple / list is the interconnect matrix K.

I would be very happy about feedback to make this connection information passing a bit more easy, this was the first idea I came up with and probably there are better ways to do it.

Future plans

This PR forms the basis for another PR I have planned soon, where I want to introduce a ModularPHReductor, which gets different PHLITModel, reduces each with a specified reductor and then interconnects the reduced models back together. For this interconnection, I require the from_interconnection in this PR (and I think it deserves its own PR that why I did not include the modular reductor changes in here)

TODO

• add tests
• maybe rewrite some code in some examples / demos using this method to show how i can be leveraged