Draft: Visualization: crosses/dots on non-significant grid points
MR for the developments discussed in #196:
- New parameter in recipe to configure which grid points to mark. Non-significant by default.
- In metrics plots, points are masked when they are NOT statistically significant
- In forecast, most likely terciles and extremes, points are masked when RPSS is negative
- Crosses drawn by default (keeping the dots in the output_size regions, to not break compatibility).
- Caption explaining what grid points are being marked.