Move CSV-related code into submodules of `input`

[alexdewar]

Currently, code for reading in data from CSV input files is spread across files for the different data structures (e.g. agent.rs, process.rs etc.) and it would be more readable if this input-related code was separated out. Following a discussion, we decided the best route was to make submodules under input: e.g. you'd have input/agent.rs for the agent CSV reading code etc.

Note that as much as possible, we want to not make functional changes at the same time as doing this refactor, as they'll be lost in the large diffs.

Subissues:

• agent.rs: Move CSV-related code to input/ #282
• asset.rs: Move CSV-related code to input/ #283
• commodity.rs: Move CSV-related code to input/ #284
• demand.rs: Move CSV-related code to input/ #285
• process.rs: Move CSV-related code to input/ #286
• region.rs: Move CSV-related code to input/ #287
• time_slice.rs: Move CSV-related code to input/ #288