Check that no commodity is both an input and an output of a process

[tsmbland]

As it says in the title. We should probably do this before #165, as I think any solution for that one will assume that no commodity is both an input and an output of a process

[tsmbland]

I think more generally, since inputs and outputs are in the same table (inputs are negative, outputs are positive), we need the restriction that no commodity appears more than once for each technology, regardless of sign (e.g. we don't want a commodity appearing twice as an output either).

[tsmbland]

There's currently no validation for the ProcessFlow table, so I first need to add a validation framework before any validation steps can be added. In line with the other tables, I'll need to create the following:
ProcessFlowRaw
read_process_flows_from_iter
read_process_flows