Allow special aesthetic for relative positioning

[OriolAbril]

Not very clear on what we want this to be. I think a good starting point would be allowing a yrelative (or some better name) in plot_dist which is then converted into an actual y value multiplying by the maximium of the kde/hist/ecdf inside plot_dist.

Idea of steps involved:

1. User provides yrelative values
2. aesthetics are generated for yrelative which can be retrived as dataset
3. yrelative dataset is multiplied by density.max(some_dims) -> xarray handles all aligment and broadcasting
4. y aesthetic is created and yrelative deleted or yrelative gets its values overwritten and the aesthetic renamed (whatever we find easier to implement)
5. Plotting backends only see y aesthetic

This might not 100% work with the exact relative ratios we'd want but I think it will generally be good enough and probably much easier than attempting to handle relative spacing across backends. But it is also true I have literally no idea how to use relative units in bokeh nor plotly, so it is always possible that is easier than the steps above.

[aloctavodia]

I agree in general, but just providing y may be enough, as we then scale them by max density. At least for plots like plot_dist, it makes sense to always think of y in relative terms.