Simplify Meridional Heat Transport notebook

[adele-morrison]

The Meridional_heat_transport.ipynb notebook currently calculates heat transport by 3 methods. Two of those methods are clearly incorrect, as shown in the figure from the notebook, copied here. The blue line is the correct method using the online heat transport diagnostic, and the orange and green are incorrect methods (that are wrong by a factor of ~2 in the Southern Hemisphere!):

I suggest we simplify this notebook and delete the incorrect methods 2 and 3, because we don't want to encourage people to compute incorrect metrics. We should encourage people to output the correct diagnostics, not try to hash together incorrect answers after the fact.

Am I missing something obvious about why we need these?