Set default color in plot_dist

[OriolAbril]

Right after

arviz-plots/src/arviz_plots/plots/distplot.py

Line 229 in 6f8b750

density_dims, _, density_ignore = filter_aes(plot_collection, aes_map, kind, sample_dims)

we should check if color has an aesthetic mapped to it and if not srt a default color. Otherwise due to matplotlib default property cycle we get lines with different colors without any aesthetic mapping.

We already do something similar for plot_forest: https://github.com/arviz-devs/arviz-plots/blob/main/src/arviz_plots/plots/forestplot.py#L489-L493

[OriolAbril]

closed by #47