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 Job ID Name Coverage
  Static Checks
passed #1399517
check_no_asserts

00:00:13

 
  Build Docker
passed #1399518
build_docker_from_cache

00:00:14

 
  Test
passed #1399520
test_mpi

00:00:32

passed #1399519
test_serial

00:09:23

85.0%
 
  Demo Runs
passed #1399528
run_bernoulli

00:00:32

passed #1399529
run_curve_fitting

00:00:30

passed #1399527
run_getting_density

00:08:44

passed #1399523
run_getting_started_1

00:00:37

passed #1399524
run_getting_started_2

00:00:57

passed #1399525
run_getting_started_3

00:06:04

passed #1399526
run_getting_started_mf

00:07:57

passed #1399521
run_ipynb0

00:01:32

passed #1399522
run_ipynb1

00:01:37

passed #1399531
run_meanfield

00:01:39

passed #1399530
run_visual_vi

00:11:07