Add variable PSF option to romanisim and make it the default. #487

Job	Run time
Download data	11s
check / Load tox environments	3s
check / check-style (ubuntu-latest)	14s
test / Load tox environments	4s
test / py39-oldestdeps-cov-xdist (ubuntu-latest)	8m 46s
test / py39-xdist (ubuntu-latest)	7m 50s
test / py310-xdist (ubuntu-latest)	7m 35s
test / py3-xdist (ubuntu-latest)	8m 2s
test / py3-xdist (macos-latest)	9m 17s
test / py3-pyargs-xdist (ubuntu-latest)	7m 49s
test / py3-cov-xdist (ubuntu-latest)	9m 10s
	59m 1s

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