Add option to change size of benchmark tests

This PR adds the variable BENCHMARK_LEVEL to allow users to change the size of benchmark tests that are run. By default BENCHMARK_LEVEL=1, so

runs small tests that use the input files with the _level_1 suffix. These should each run for a few seconds on a few cores (1 to 4 ish).

Bigger tests can be run with

These use the input files with the _level_2 suffix, and are designed to run for a few minutes on around 24 to 96 cores.

Note that the user still has to select the number of cores themselves using the NTESTPROCS variable.

I’m running the level 2 input files on SCARF, but want to tweak the input files so that the tests don’t take too long, but still run long enough that code improvements can be sensibly measured. The original state of this PR produced data in this job:

https://anvil.softeng-support.ac.uk/jenkins/job/Gyrokinetics/job/scarf/29/

I’ll summarize it in a table below.