Question about processes with no output commodities

It's currently possible to have processes with no output flows. I can imagine a process with no outputs in real life (e.g. carbon capture), but since investments in MUSE are driven by demand for outputs (namely the _primary_ output), there's currently no way for a process like this to be invested in.

Additionally, if a user included an asset like this in the input data, it would get decommissioned immediately in the first investment year, because we've set things up so that agents have to _opt in_ to retaining assets, and there's currently no way to do this for processes with no primary output.

Perhaps there's a longer term plan for this, but I haven't really seen it discussed so thought I'd mention it.

Until we have a solution, perhaps it's better to disallow processes with no outputs?

@ahawkes thoughts?