Add BSM2 layout

[yalinli2]

Open a draft PR for discussion. Just added the layout and made sure the system can be created (using the most basic configuration in the report (and praying they have this implementation in MATLAB)

Note: you need to use the metro branch of QSDsan, although the unit operations need some cleanup

Didn't dare to let it simulate at all, haven't looked at the parameter settings, etc. But first need to figure out:

1. Can I pretty much just copy the entire bsm1 layout into bsm2?
2. There's a storage tank in the config that we don't have, using HydraulicDelay for now.

Also not about BSM2 but BSM1 - does it make sense to run BSM1 using ASM2d? I made some changes in the bsm1 module to allow this, but it might make no sense at all.

And on DiffusedAeration, would be nice to store DO_ID so that when creating aerobic tanks, we can just get the DO_ID from the process.

[yalinli2]

3. Where is influent TSS used?