D1.1 Migrate Method to use processed arrays directly

[cmutel]

We don't need to, or even necessarily want to, store all CFs in the database. Instead, taking inspiration from the read-only proxies in the IO table backend, we can interact directly with the processed arrays of characterization factors. This has several advantages:

• Very fast import of methods when setting up projects
• Very fast import of methods provided in the lciafmt specification
• Don't need to figure out how to store more complicated CFs with uncertainty of scenarios in SQLite

Tasks:

• Update bw2io.convert_lcia_methods_data to product a lciafmt dataframe
• Design the extra metadata fields for storing LCIA data (e.g. Context separator character, how bibliographic data is stored)
• SQLite row schema for methods. Optional inclusion of dataframe in database.
• New bw2io importer for lciafmt methods