mesh.generation: port away from simplex_onb #99

Job	Run time
Examples Conda Py3	4m 14s
Documentation	4m 0s
Tests for downstream project grudge	9m 4s
Pytest Firedrake	6m 33s
Tests for downstream project pytential	43m 25s
Examples Firedrake	3m 21s
Flake8	19s
Tests for downstream project mirgecom	4s
Tests for downstream project mirgecom_examples	3s
Mypy	3m 1s
Pylint	3m 1s
Pytest Conda Py3	11m 0s
	1h 28m 5s

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