Work toward pedantic Clippy lints.

[jetuk]

Suggested process:

• Merge fix: Fix clippy warnings in clp-sys crate. #109, fix: Fix some clippy warnings in pywr-core crate. #110, and fix: Fix clippy warnings in pywr-schema crate. #111
• Fix any remaining lints
• Fix some lints produced by cargo clippy -- -W clippy::pedantic. There are currently more than 1000!
• Add #![warn(clippy::pedantic)] to relevant lib.rs or main.rs files.
• Add #[allow] to permit some lints we think are not useful to address or false positives.