Add a `meshio` reader #2332

Job	Run time
Examples Conda Py3	4m 6s
Pytest Firedrake	6m 39s
Tests for downstream project grudge	9m 6s
Documentation	3m 59s
Tests for downstream project pytential	43m 16s
Examples Firedrake	3m 36s
Flake8	16s
Tests for downstream project mirgecom	4s
Mypy	2m 49s
Tests for downstream project mirgecom_examples	6s
Pylint	2m 59s
Pytest Conda Py3	11m 0s
	1h 27m 56s

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