add warning scatter plot

[daniel-caichac-DHI]

Hi, I added a warning the following reasons

• Usually when plotting a scatter plot with points, having 50k points is enough, and for larger datasets by default a sample of the dataset is plotted (as this is faster than plotting the complete dataset)
• In most cases this is ok, but sometimes the user wants to see all points to identify the possible outliers, this was already possible with show_points=True , but sometimes the user (aka myself) forget about this.
• I rarely have more than 50k points when comparing model/measurements, but when I do, I want to be reminded that the plot is just using a sample, so I added a warning that, if a sample of the total set is being plotted (despite the statistics being calculated on the total data set), will remind the user of this
• For smaller datasets (<50k points) the total set is plotted and the warning is not triggered. Also if show_points=True or show_points=1.0 the warning is not triggered.

[jsmariegaard]

Great idea - looks good