Fix misaligned demand curve in tutorial 8 power plot

[patrickbryant]

In playing with tutorial 8 I noticed that demand seemed to exceed total generation in some hours of the power plot. This is due to the Demand_data.csv time series starting at row 2 while in the power.csv the timeseries starts at row 4.

The only code change is in the 6th block. I added some comments to explain the row indexing and shifted loads_tot up by 2 rows:

tstart = 3 # time series starts at row 3 in results files
...
loads =  CSV.read(joinpath(case,"TDR_results/Demand_data.csv"),DataFrame,missingstring="NA")# if not using Time-Domain Reduction use system/Demand_data.csv
...
power_plot[!,"Demand_Total"] = repeat(loads_tot[tstart-2:tend-2],4); # Demand_data.csv timeseries starts at row 1

I think it is worth fixing this for beginners like me; when I noticed the gaps between generation and demand I thought something might have gone wrong in the run resulting in unserved energy, or that I was misunderstanding something in the plotted data.

P.S. I'm a fellow at the US Department of Energy; I manage some projects which use various macro energy system models. I am finally making some time to learn to use one myself. Without a GAMS (CPLEX) license I can't effectively use ReEDS so I am very glad GenX exists! I hope to contribute from time to time as I go through the process of playing and learning. Don't hesitate to tell me if I'm doing things wrong or causing you a headache.