Processes: Combine PACs and flows in code and CSV file structure

[alexdewar]

All processes have commodity flows, some of which are PACs (Primary Activity Commodities). Currently the Process struct has separate flows and pacs fields (the latter just being a Vec<Rc<Commodity>>. This reflects the structure of the input files: there are separate process_flows.csv and process_pacs.csv files. It would be better to just indicate whether a commodity flow corresponds to a PAC with a boolean flag (is_pac, maybe). At the same time, we should also get rid of the process_pacs.csv file, which should mean we can delete some code too.

Given that we will likely want to iterate over a Process's PACs quite often, it would be useful to have a helper function for this. Something like:

impl Process {
    pub fn iter_pacs(&self) -> impl Iterator<Item = &ProcessFlow> {
        self.flows.iter().filter(|flow| flow.is_pac)
    }
}

Note: Implementing this will also probably fix #338. Please assign yourself to that issue too if you take this one and verify that you have fixed the bug.