Add benchmark script python/test/test_benchmark.py, which calls get_symmetry_dataset for all cells in python/test/data. This benchmark is not performed as default (--benchmark-skip).

Here is an example of the benchmarking:

pytest --benchmark-only --benchmark-columns=mean,stddev -s -v test/test_benchmark.py
test/test_benchmark.py::test_get_symmetry_dataset Benchmark get_symmetry_dataset on 784 structures
PASSED
----- benchmark 'space-group': 1 tests -----
Name (time in s)                Mean  StdDev
--------------------------------------------
test_get_symmetry_dataset     1.9893  0.0296
--------------------------------------------

The following changes are also added though they do not relate to the issue directly:

• Add extra_require for pytest: pip install -e .[testing]
• Fix typo in myst setting
• Add developer docs for benchmark: doc/development/performance.md
• Format layer-group implementation (accidentally)