Further investigation into in-sewer decay

[barneydobson]

• Decays "should" be reasonably straightforward to add by converting the sewer QueueTank to a DecayQueueTank. (See https://github.com/barneydobson/wsi/blob/main/wsimod/nodes/storage.py /QueueGroundwater for an example of how to do this in an accommodating way).
• Main issue is very little seems to be known about in-sewer decay, even the processes involved, let alone parameterisation. Probably this needs a full study.