✨ gains numeric sum_with_threads

[jibarozzo]

This PR enhances the functionality of the sum_with_threads function to support both integer and double numeric types.

Initially, the approach of handling both types via NumericSexp was considered, but this was not feasible due to the fact that NumericSexp does not implement the to_vec() method, which is necessary for the operation. Following the compiler suggestions, the solution was to use a match expression within sum_with_threads to explicitly handle two distinct types of numeric data: IntegerSexp for integers and RealSexp for doubles. This enables the function to process both types separately via two implementation.

[jonocarroll]

I don't know where better to put this - I was curious about the difference between thread and rayon and had a go at building a comparison myself. Rayon seems easier to set up, unless you want to specify the number of threads and re-use a thread pool, but even then it wasn't so bad.

What surprised me was that compared to just doing x.iter().sum() it's a lot more overhead and it's not clear that it's worth it, at least for the 'sum this random data' test case I implemented

> cargo run --release

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Sum with single thread: 56.96µs
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Sum with 2 threads: 229.21µs
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Sum with rayon (2 threads): 173.04µs
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Sum with 4 threads: 185.79µs
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Sum with rayon (4 threads): 101.29µs
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Sum with 8 threads: 229.67µs
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Sum with rayon (8 threads): 276.13µs

Maybe I implemented something wrong - I haven't played with multithreading in Rust before, so this is a great opportunity for me to learn.

[jdhoffa]

One tiny NIT comment on style, but functionally looks great!

I tested this by:

• Reviewing the added unit test to ensure we are testing for the new behavious
• Syntactically reviewing the code
• Pulling and building locally, and testing out the functionality in a live R session

[jdhoffa]

@jonocarroll I extracted your comment into this discussion post for posterity so that we don't lose track of it once this PR is merged 😊 I will respond there.