Add univariate KDE

[sethaxen]

This PR implements univariate KDE, following #6, as well as a cumulative variant.

It tries to make as few modifications to KernelDensity as possible, in the hopes of upstreaming this code to KernelDensity. Its extra functionality is limited to:

1. padding the density bounds for convolution by n histogram bins, where n is chosen such that the probability mass in the left padding area from convolving the kernel with the rightmost data point is less than 1e-3. This padding is removed, which without boundary correction would produce an unnormalized KDE
2. performing boundary correction using reflection. We double the padding and reflect the density in the half of the padding closest to the data boundaries, which is equivalent to reflecting the data points to fill the half of the padding closest to the data boundaries. When all padding is removed, this produces a normalized KDE.

It differs from Python arviz primarily in when the boundary correction is performed. Python ArviZ performs it after binning the data into histograms but before convolution. We perform it after convolution.

The two methods are numerically very similar, though the maximum density of this implementation always seems to be a little higher than Python ArviZ's.

EDIT: removed the cumulative KDE functionality because it's somewhat trivial to get from the KDE and not really recommended, since the ECDF is superior for visualization.

[sethaxen]

The numerical issues appear to be completely resolved:

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 43 lines in your changes missing coverage. Please review.

Project coverage is 92.82%. Comparing base (d4597db) to head (34af628).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
src/kde.jl	0.00%	43 Missing ⚠️

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Flag	BASE (d4597db)	HEAD (34af628)
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
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- Coverage   99.82%   92.82%   -7.01%     
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  Files          11       12       +1     
  Lines         570      613      +43     
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  Hits          569      569              
- Misses          1       44      +43     

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