[Feature Request] Ability to have multi layer first walls

[Allentro]

I think it would be a useful addition to have multi layer firstwalls. This would enable more realistic constructions which often include plasma facing material, thermal conducting interlayer and a structural layer.

[shimwell]

This sounds like a great idea to me. I'm thinking it is such a nice addition that it could be expanded to apply to all layers.

Currently the reactors tend to have 3 layers but if the could have n layers that would be great.

It will certainly allow a larger variety of reactors to be made but it might also shorten the amount of code.

It would effectively turn all the reactors into onion reactors.

I say we merge in the PR you have kindly opened but perhaps consider refactoring the reactor layer creation more radically