Add tracing support

[jetuk]

Using the tracing crate would be a good way to add some effective debugging & logging support to Pywr. I think it'll be more powerful than simple logging. E.g. we can trace each Parameter calculation for debugging purposes.

https://github.com/tokio-rs/tracing

I am not sure how well this would integrate with Python's logging system.

[jetuk]

Swapped out log for tracing in #48. Now instrumentation needs to be added.

All parameter calculations could be instrumented at the "trace" level for low-level debugging. Each time-step at the "debug" level. These will be helpful if they can include the model's state in the span, but that might be a lot of data to include.

[jetuk]

#52 added a subscriber to the Rust and Python CLI. Long-term I think this will need to be slightly refactored:

• Only Rust CLI to depend on the tracing subscriber (complete in feat: Make tracing-subscriber a dependency of pywr-cli only. #117).
• Python extension to use some method allow the tracing output to be controlled by the Python logging module.