Tidy up the cost classes to avoid repetition #560

Job	Run time
style	11s
Coverage tests (macos-14 / Python 3.12)	18m 24s
Benchmarks	7m 12s
Integration tests (ubuntu-latest / Python 3.12)	24m 48s
Test examples (ubuntu-latest / Python 3.12)	24m 10s
Integration tests (windows-latest / Python 3.12)	24m 46s
Test examples (windows-latest / Python 3.12)	26m 16s
Integration tests (macos-13 / Python 3.12)	38m 44s
Test examples (macos-13 / Python 3.12)	28m 26s
Integration tests (macos-14 / Python 3.12)	10m 27s
Test examples (macos-14 / Python 3.12)	11m 17s
Unit tests (ubuntu-latest / Python 3.9)	1m 57s
Unit tests (ubuntu-latest / Python 3.10)	1m 56s
Unit tests (ubuntu-latest / Python 3.11)	1m 51s
Unit tests (ubuntu-latest / Python 3.12)	1m 59s
Unit tests (windows-latest / Python 3.9)	2m 54s
Unit tests (windows-latest / Python 3.10)	3m 44s
Unit tests (windows-latest / Python 3.11)	3m 11s
Unit tests (windows-latest / Python 3.12)	3m 53s
Unit tests (macos-13 / Python 3.9)	3m 43s
Unit tests (macos-13 / Python 3.10)	2m 30s
Unit tests (macos-13 / Python 3.11)	4m 21s
Unit tests (macos-13 / Python 3.12)	4m 6s
Unit tests (macos-14 / Python 3.9)	2m 30s
Unit tests (macos-14 / Python 3.10)	1m 29s
Unit tests (macos-14 / Python 3.11)	1m 5s
	4h 15m 50s

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