Add types for _merge_boundary_adjacency_groups #91

Job	Run time
Mypy	3m 5s
Pytest Firedrake	6m 28s
Flake8	16s
Pylint	3m 0s
Pytest Conda Py3	10m 47s
Documentation	3m 51s
Examples Conda Py3	4m 6s
Tests for downstream project grudge	9m 14s
Tests for downstream project pytential	43m 15s
Tests for downstream project mirgecom	6s
Examples Firedrake	3m 23s
Tests for downstream project mirgecom_examples	6s
	1h 27m 37s

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