Review conc indexer

[yalinli2]

Hey @joyxyz1994 I think I managed to implemented the concentration indexer through 87aec8fa9ad0b84951ae5579bb57ea1842953df4

import qsdsan as qs
qs.set_thermo(qs.Components.load_default())
ws1 = qs.WasteStream.codbased_inf_model(flow_tot=100)
ws1.conc
ws1.iconc['S_F']

Can you take a look at the changes to _waste_stream.py and see whether it makes sense? I think with it we might not need the get_mass_concentration method (or let it just return self.conc)

Thanks!

[yalinli2]

It's already in use, I'm closing it now!