WIP: Better multi core

[calum-chamberlain]

What does this PR do?

1. Turns outer-core threading back on (this appears to be stable on my machines)
2. Uses larger FFT lengths than previously set as default - this gives better performance over a wider range. Users still have the option to change the fft length.

Why was it initiated? Any relevant Issues?

• Large multi-core machines (>40 threads) were not being well-loaded by version 0.4.0.

PR Checklist

• [x] develop base branch selected?
• [x] This PR is not directly related to an existing issue (which has no PR yet).
• [ ] All tests still pass.
• [ ] Any new features or fixed regressions are be covered via new tests.
• [ ] Any new or changed features have are fully documented.
• [ ] Significant changes have been added to CHANGES.md.
• [ ] First time contributors have added your name to CONTRIBUTORS.md.