feature: heatmap years to svg layers

[yeKcim]

when drawing heatmap with several years, separate each year in layers could be great (same scale, can enable or disable easily in svg editor…)

[Florian Pigorsch]

I'm now putting everything into separate groups:

• The header and footer areas are but into the groups "header" and "footer".
• The main drawing area is put into the group "tracks".
• Individual years (inside the main drawing area) are put into groups "yearXXXX".

I hope this helps...