Enhance MapAggregator

[JaskRendix]

Waiting #62

PR improves the MapAggregator class:

• Layering maps: you can now give each map a "z-index", so you can control which maps appear on top of others
• Removing maps: you can now take maps out of the aggregator if you don't need them anymore
• Tile size check: the code now makes sure all the maps you add have the same tile size, to prevent weird display issues
• Fixes: some minor fixes and improvements to how the maps are positioned and sized
• Unittest suite MapAggregator and the other two classes in data.py