Check that all PACs for a process are inputs or outputs

[alexdewar]

The PACs for each process must be all inputs or all outputs (see #136).

It would probably be best to implement this in read_process_pacs_from_iter() in process.rs. Once we've generated the HashMap of process PACs, at that point we'll be able to check whether they're all inputs or outputs for each proceses.

[tsmbland]

We also need to check that no commodity is both an input and an output of a process (or at least I assume that shouldn't happen - we've had major problems in MUSE1 from people accidentally doing this)

[alexdewar]

That makes sense. Should we make a separate issue so we don't forget about it?