Add option to change spatial point color if location is deemed "significant"

[pinarda]

based on zscore or annual_harmonic_relative_ratio

[pinarda]

calculate cutoff (see single_metric cutoff/%significant metrics), compare each point to cutoff and change values at these points to NaN or something (or just change color without changing values if possible).

Maybe we could set the colorbar limits to everything that is not significant, then set_over color to a bright color