Add copyright statement to all source files #937

Job	Run time
linux/3.12	16m 47s
linux/3.8/bare-env	8s
osx/3.10	27m 5s
lint/style-and-typos	45s
win/3.9	37m 53s
linux/3.11/conda	20m 28s
linux/3.9/mpi	8m 56s
linux/3.10/cython	19m 10s
win/3.8/pip	26m 43s
process-coverage-${{ matrix.TARGET }}	1s
	2h 37m 56s

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