Introduce weights keyword to plot_kde

[jvaara]

Certain Approximate Bayes Computation packages provide samples with weights; it would be useful to be able to use plot_kde directly for these applications.

At least for the stats.density_utils._fast_kde_2d, it would be a straightforward modification.

Cheers

[tomicapretto]

Given how the KDE is implemented in ArviZ, using a grid of points, we first need to consider how to account for the weights in the binning. This seems to be already solved in KDEpy https://github.com/tommyod/KDEpy/blob/master/KDEpy/binning.py