Generic Parameter trait

[jetuk]

Some tasks to reduce some duplication in pywr-core.

• Make Parameter trait generic over the calculated value type. refactor: Make the Parameter trait generic over the calculated value type #134
• Use a single ParameterIndex that is also generic and tied to the calculated value type. refactor: Make ParameterIndex generic #138
• Remove as_any_mut and corresponding dead code in pywr-core/src/parameters/profiles/monthly.rs. I think as_any_mut is only needed for this experiment that is now redundant. chore: Remove Parameter::as_any_mut #139