Add region_id to ProcessFlow

[tsmbland]

Following on from #1311, this adds a region_id field to ProcessFlow so we can (in theory) represent flows between regions. More still to do (probably in separate PRs):

• the inner map in ProcessFlowsMap is keyed by CommodityID, but we'll probably need to key this by (CommodityID, RegionID) to allow for multiple entries for the same commodity (i.e. out of one region and into another), or store as a vec
• the graph code currently doesn't allow for the possibility of flows between regions
• a way to build trade processes via the input files
• a way to represent cross-region flows in the output data
• possibly more?

It's quite fiddly though and I'm a bit uneasy about this. We need to switch all flow-related stuff that's currently relying on the asset's region to rely instead on the ProcessFlow region, but there's no compiler safety to protect against mistakes so will require a lot of care.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.71%. Comparing base (6723361) to head (e6263cb).

Additional details and impacted files

@@              Coverage Diff              @@
##           flows_map    #1314      +/-   ##
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+ Coverage      89.70%   89.71%   +0.01%     
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  Files             58       58              
  Lines           8481     8492      +11     
  Branches        8481     8492      +11     
=============================================
+ Hits            7608     7619      +11     
  Misses           566      566              
  Partials         307      307              

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[alexdewar]

@tsmbland I know we talked about this, but I'm wondering if this is actually the right approach. Generally we only use ProcessFlows to get the coeffs (sometimes filtering depending on whether it's an input or output flow).

When we add trade, there will be two categories of process flows:

• Flows for assets: coeff is a number between -1 and 1, region is always the same as the asset's
• Flows for "trade processes": coeff is either -1 or 1, with region being the input or output region, respectively

I'm just wondering whether the ProcessFlow struct is the right place to put this info.

Another way to go might be to add a getter to our two kinds of asset-like objects, e.g.:

pub fn get_primary_output_flow_coeff(commodity_id: &CommodityID, output_region: &RegionID) -> Option<Flow>;

I'm kind of thinking on my feet here, but you get the idea.

[tsmbland]

Potentially! I'm also thinking on my feet and just wanted to see what this would look like. Based on recent correspondence with Adam I was kind of thinking that we'd inevitably have to represent trade processes as regular assets just like any other, so this seemed inevitable