Add heatmap to MTMatrix

[DurieuxPol]

You can now generate a heatmap for MTMatrix.
It groups the mutations by operator and the tests by class, and represents the percentage of the mutations from a given operator that are killed by the tests from a given class.

matrix := MTMatrix forClasses: { UUID } andTests: {
		          UUIDTest.
		          UUIDPrimitivesTest }.
matrix build.
matrix generateHeatmap

[guillep]

Oh this is super sexy! :D
Could I ask to go from yellow 🟨 to red 🟥 instead of light-to-dark blue?