Add pymatgen writer #268

Job	Run time
build (ubuntu-latest, meson, debug, gnu, 12)	31s
build (macos-latest, meson, debug, gnu, 12)	43s
build (ubuntu-latest, meson, debug, gnu, 10)	1m 46s
build (ubuntu-latest, fpm, debug, gnu, 12)	42s
build (ubuntu-latest, cmake, debug, gnu, 12)	34s
build (ubuntu-latest, meson, debug, intel, 2021)	2m 27s
build (macos-latest, cmake, debug, gnu, 12)	48s
build (ubuntu-latest, meson, coverage, gnu, 11)	1m 42s
	9m 13s

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