Tutorial 8.2 - over-investment with EAC objective and problems with weighted sum

[tsmbland]

A couple of bugs in the model are causing this tutorial to give nonsensical results, specifically:

• Unexpected behaviour with the weighted_sum decision method [BUG]  #317
• Negative EAC leads to over-investment #319

Once these bugs are fixed, we will need to return to this tutorial and make sure everything looks reasonable