Improvements to benchmarking system

[matt-graham]

A collection of improvements to benchmarking system made during the DiRAC NVIDIA hackathon

• Due to behaviour noted in NumPy generate_precomputes implementation has cubic in L memory overhead #246 switches to always using JAX implementation for generating precomputes for spherical transforms to avoid cubic in L overhead.
• Adds benchmark modules for separate precompute versions of transforms.
• Some improvements to help text of benchmarking scripts
• Adds option to run benchmarked functions once initially without recording time to remove effect of initial JIT compilation from reported times
• Fix to issue causing only last parameter set results being recorded in JSON output
• JSON output now records additional metadata about host system such as installed package versions and information about GPUs and CPUs available, to make these more useful artefacts to store for longer term comparison.