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 Job ID Name Coverage
  Static Checks
passed #1399532
check_no_asserts

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  Build Docker
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build_docker_from_cache

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  Test
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test_mpi

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test_serial

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  Demo Runs
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run_bernoulli

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run_curve_fitting

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run_getting_density

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run_getting_started_1

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run_getting_started_2

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run_getting_started_3

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run_getting_started_mf

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run_ipynb0

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run_ipynb1

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run_meanfield

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run_nonlinearity_guide

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run_visual_vi

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