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.
16 jobs for fix_de_key7 in 15 minutes and 14 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Static Checks
passed check_no_asserts #1399532

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  Build Docker
passed build_docker_from_cache #1399533

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  Test
passed test_mpi #1399535

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passed test_serial #1399534

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85.0%
 
  Demo Runs
passed run_bernoulli #1399543

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passed run_curve_fitting #1399544

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passed run_getting_density #1399542

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passed run_getting_started_1 #1399538

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passed run_getting_started_2 #1399539

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passed run_getting_started_3 #1399540

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passed run_getting_started_mf #1399541

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passed run_ipynb0 #1399536

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passed run_ipynb1 #1399537

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passed run_meanfield #1399546

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passed run_nonlinearity_guide #1399547

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passed run_visual_vi #1399545

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