Add a way to dampen pollution

[graememcc]

Once you introduce an airport or port, you are going to be stuck with high pollution messages ad nauseum. Perhaps we should introduce a mechanism where some tiles (e.g. parks) mitigate the pollution.

[Quix0r]

Most city builders use trees for this. But please aware that trees are no perfect air filters. This means that the user should add some levels of trees around airports / seaports to reduce pollution.