need legend on the emissions plot

[cboisvenue]

The CBMutils::carbonOutPlot() fnct plots emissions as one line per gas (CH4, CO, CO2), and the bars for products on the same graph with two y-axis. We need a legend for the lines. Can't get the legend to appear in ggplot2.

[cboisvenue]

Plots and output have been update in the RIA implementation of spadesCBM. Legends are not needed anymore, but the main branch needs to be updated with the improvements from the RIA project.