Add grass to Wolf-Sheep Predation Model

[dmasad]

The wolf-sheep predation model does not currently have the sheep eating grass. Add in the grass, and have it reduced by sheep eating it, and grow back at a fixed rate.

[davpaez]

I'm working on this ;)

[dotsdl]

@davpaez are you still working on this? Otherwise, happy to dig into it.

[jackiekazil]

@dotsdl, I assume you were at the sprint? Would you still be interested if @davpaez does not chime in soon?

[dotsdl]

Yup. Still interested. :D

[davpaez]

Hi, I'm sorry for the delay. I just submitted a pull request. It seems to be working. I'd like to hear your toughts on the custom schedule that I added.

[dmasad]

@davpaez are you still working on the changes to the pull request, and/or do you want someone else to jump in and work on it?

[jackiekazil]

Let's move this convo to #169