Add code coverage using codecov-action. #566

Job	Run time
mmg-debug / ci (ubuntu-20.04, on, off, on, off, int32_t)	1h 16m 56s
mmg-debug / ci (ubuntu-20.04, on, off, off, off, int32_t)	1h 37m 14s
mmg-debug / ci (ubuntu-20.04, off, off, on, off, int32_t)	1h 2m 54s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int32_t)	1h 11m 8s
mmg-debug / ci (macos-12, on, off, on, off, int32_t)	42m 54s
mmg-debug / ci (macos-12, on, off, off, off, int32_t)	54m 9s
mmg-debug / ci (macos-12, off, off, on, off, int32_t)	33m 2s
mmg-debug / ci (macos-12, off, off, off, off, int32_t)	55m 5s
mmg-debug / ci (ubuntu-20.04, off, off, off, on, int32_t)	3m 35s
mmg-debug / ci (macos-12, off, off, off, on, int32_t)	12m 49s
mmg-debug / ci (ubuntu-20.04, off, on, on, off, int32_t)	1h 3m 50s
mmg-debug / ci (macos-12, off, on, on, off, int32_t)	48m 27s
mmg-debug / ci (windows-2022, off, off, off, off, int32_t)	54m 21s
mmg-debug / ci (windows-2022, on, off, off, off, int32_t)	54m 7s
mmg-debug / ci (windows-2022, off, on, off, off, int32_t)	55m 11s
mmg-debug / ci (ubuntu-20.04, on, on, on, on, int64_t)	10m 7s
mmg-debug / ci (ubuntu-20.04, off, off, on, on, int64_t)	1h 10m 31s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int64_t)	2m 5s
mmg-debug / ci (macos-12, on, on, on, on, int64_t)	7m 40s
mmg-debug / ci (macos-12, off, off, off, off, int64_t)	3m 35s
mmg-debug / ci (windows-2022, on, on, off, off, int64_t)	4m 55s
mmg-debug / ci (windows-2022, off, off, off, off, int64_t)	4m 11s
	14h 48m 46s

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