[1.1.55] Fluid systems can mix using underground pipes

[Waity5]

Here's a 3 step process for mixing fluid systems:
Imgur link because I'm not sure how images are meant to work

[boskid]

Nothing special, there are many ways fluids can mix. Because the hard fluid mixing prevention logic was discarded, we allow fluids mixing. There are only couple of interactions when fluid mixing check is done and this is one of them but this is a weird corner case where a pipe to be built does not see one of the connections because it is already connected to the entity being replaced. I do not care about this case.