Allow for adding commodity constraints

This information lives in `commodity_constraints.csv`.

Here are the suggested column headings for the CSV file from the schema:

```csv
commodity_constraint,commodity_id,region_id,limit_type,balance_type,year,time_slice,value
...
```

More info here: https://lucid.app/lucidchart/6d02ff7b-35ba-4e90-97ec-b695ff68b6e1/edit?page=0_0&invitationId=inv_ab477d33-772d-476f-b2e8-7d4cac7c90cf#

**However**, I think we may want to change this a bit. For one thing, I think we should do away with the `limit_type` column and have a range of values instead, like we're planning for process availabilities (#363).

Also, as Adam has said, there are implications for this outside of dispatch:

> Initially a dispatch issue, but also has implications for investment appraisal where investment signals may be positive for a candidate asset, but it cannot be used in dispatch due to a commodity limit. Vice versa, a candidate asset that is needed to achieve of commodity requirement may have negative investment signals.