density_estimator: Handle an optional CF prefix

Accepting a prefix allows using the density estimator within other
models without having to worry about conflicting keys.
15 jobs for fix_de_key in 21 minutes and 26 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Static Checks
passed check_no_asserts #1399517

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  Build Docker
passed build_docker_from_cache #1399518

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  Test
passed test_mpi #1399520

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passed test_serial #1399519

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85.0%
 
  Demo Runs
passed run_bernoulli #1399528

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passed run_curve_fitting #1399529

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passed run_getting_density #1399527

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passed run_getting_started_1 #1399523

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passed run_getting_started_2 #1399524

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passed run_getting_started_3 #1399525

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passed run_getting_started_mf #1399526

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passed run_ipynb0 #1399521

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passed run_ipynb1 #1399522

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passed run_meanfield #1399531

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passed run_visual_vi #1399530

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