Add DistributionPlot and remove GKDE-plot

DistributionPlot implementation

Removed GKDE, including references in plot_window-tests

Added DistributionPlot and corresponding tests. The plot implements
three methods of plotting the distribution:
- Gaussian KDE
- Overlapping Histogram (y-axis can be set to count or density)
- Rug plots for to visualize the raw datapoints.

Behaviour of the plot changes depending of what plots are toggled.
If GKDE or Hist. is turned off, the rugplots will act as the main window
otherwise, they will be a supporting plot underneath the "main" plot-window

If multiple ensembles are selected the overlapping histograms can become muddied
and hard to distinquish. This is not a common use-case as most users will only select
first and last.

No screenshot-test added, only unit testing. Plot will likely undergo refactoring/updates,
so will await screenshot-testing until "stable".

Note: Cannot remove original HistogramPlot as of now, as the new plot can not
handle categorical data yet.

Author: eilskra

src/ert/gui/plotting/ert_plots/__init__.py

@@ -1,6 +1,5 @@
 from .cesp import CrossEnsembleStatisticsPlot
 from .distribution import DistributionPlot
-from .gaussian_kde import GaussianKDEPlot
 from .histogram import HistogramPlot
 from .misfits import MisfitsPlot
 from .statistics import StatisticsPlot
@@ -9,7 +8,6 @@
 __all__ = [
     "CrossEnsembleStatisticsPlot",
     "DistributionPlot",
-    "GaussianKDEPlot",
     "HistogramPlot",
     "MisfitsPlot",
     "StatisticsPlot",