Quickly plot marginals for parameter distribution

[odunbar]

Purpose

Closes #254

Content

• For the wide variety of possible distributions, the easiest general solution is to plot marginal histograms of distributions, so this is the approach we take.
• Adds recipe for plot(pd::ParameterDistribution)
• Adds recipe for plot(pd <: ParameterDistributionType)
• Adds unit test which plots marginals (set ENV["CES_TEST_PLOT_OUTPUT"] = true prior to ] test)
• Adds example into the priors docs page.

In future

I expect these can be improved upon in various ways, using other plot-types, or only plotting unique distributions where they are repeated etc. but for this PR I just wanted something simple to work on any case.

Example from test/

A complicated distribution:

d1 = Parameterized(MvNormal(zeros(4), 0.1 * I))
c1 = [no_constraint(), bounded_below(-1.0), bounded_above(0.4), bounded(-0.1, 0.2)]
name1 = "constrained_mvnormal"
u1 = ParameterDistribution(d1, c1, name1)

d2 = Samples([1 2 3 4])
c2 = [bounded(10, 15)]
name2 = "constrained_sampled"
u2 = ParameterDistribution(d2, c2, name2)
            
d3 = VectorOfParameterized(repeat([Beta(2, 2)], 3))
c3 = repeat([no_constraint()], 3)
name3 = "vector_beta"
u3 = ParameterDistribution(d3, c3, name3)
            
u = combine_distributions([u1, u2, u3])

plot(u)

plot(u, constrained=false)

---

• I have read and checked the items on the review checklist.

[odunbar]

NB: codecov doesn't like this because it only tests the PlotRecipes if plots are enabled (and github actions doesn't enable them)

[odunbar]

bors try

[bors]

try

Build failed:

• test_success

[odunbar]

Note that bors has succeeded on a rerun of the test suite,

it seems like an unrelated test of inflation has a slightly too-tight tolerance which is subject to failure on random-seed change - see here for the refresh:
https://github.com/CliMA/EnsembleKalmanProcesses.jl/actions/runs/4493567718

[sandreza]

The changes look sensible to me! The only thing left is to let Bors do its thing

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bors r+

[bors]

Build succeeded:

• docs-build
• format
• test_success