Modest proposal for discussion: eliminate LinearICSystem class

[sawyerbfuller]

It is not clear to me what purpose there is for a LinearICSystem. One possibility is as a biquad cascade*, but this can already be implemented by forcing the interconnected systems to be NonlinearIOSystems, and is not currently supported by LinearICSystems anyway because the constituent systems are combined into a single system with A, B, C, D matrices and signal names.

Is there any other use case for a LinearICSystem? I can't figure out what benefit there is to preserving other characteristics of an InterconnectedSystem, if such a system is always used in an equivalent way to a LinearIOSystem.

*There are cases when you don't want a combined interconnected series of systems to be tunred into a single state-space or transfer function system. For example, if you want to numerically interconnect a series of linear systems to create a biquad cascade to avoid numerical issues that crop up when combining large and small coefficients. This comes up for example when implementing a high-order pade approximation (see e.g. https://www.embeddedrelated.com/showarticle/927.php)