Heatmap cell with 0 should be white

[guillep]

The Heatmap matrix introduced in here https://github.com/pharo-contributions/mutalk/pull/79 uses a linear palette of colors. This means that low values such as 0 and 1 get very similar colors. However, 0 and 1 are very different! 0 means all mutants survived, 1 meant at least one got killed.

• [ ] We should make cells with 0 have a white color while the palette should govern the rest of the values. One possibility is to use the color: method directly
• [ ] We should make sure that the legend is correctly set.