Fix de-embedding when 2 networks are provided

[atavella]

The network operator "//" used for de-embed 1 or 2 networks is not working for exactly 2 networks.

ntwk1 // ntwk2 works
ntwk1 // (ntwk2) works
ntwk1 // (ntwk2, ntwk3) does not work

it fixes that

[FranzForstmayr]

I think we should even be more strict and raise a Exception (RuntimeError?) if more than two elements are passed for other.

[atavella]

@FranzForstmayr that makes sense to me. But, right now is raising a RuntimeWarning if more than two elements are passed and the flow continues with just the first two elements.

[FranzForstmayr]

Yes but warnings are likely to get overseen, and there's no reason to accept more than two for others.
If some existing code triggers the warning, there's probably another issue in the code which is simply not found yet.

[atavella]

It seems to me that ValueError could be more suitable based on "Raised when an operation or function receives an argument that has the right type but an inappropriate value"

[FranzForstmayr]

Thanks @atavella