Changing a plan VS Replanning

[woodbri]

In theory, we could start a plan with one herd and N paddocks and later in the plan add another herd and more paddocks to the plan. This is implied by the Grazing chart, but I don't think we can do this with the current code structure.

I think we have decided that this is basically a replan event, ie: the old plan is archived, and a new plan is created for the remained of the season given the new inputs.

From #39