Change environmental sensitivity (effect)

[TanyaS08]

Along with ramping the landscape values we can also do this with the environmental_effect term as well.

[tpoisot]

So I had a look at the code, and I want to point at another possible tweak here. Currently you're re-calculating the optimum at every step, which takes a while. So maybe what we can do instead is simply set the values at the beginning of the simulation, and allow it to manifest in the same way as the landscape does -- this is probably equivalent to what you're doing already, but I might be able to get this to work a little faster. See #13