Improve styling for hmaps to make them easier to compare

[ptormene]

• use pretty breaks with 8 classes by default (pretty breaks will automatically change the number of classes to make better looking breaks
• when many maps are loaded altogether and we want to make the corresponding layers more comparable, we could classify using the top and bottom values across all layers to style all layers from the lowest to the highest value (instead of classifying each layer based on its own limit values)