Change plot titles to use site names, not ids

[lindsayplatt]

As @elisehinman pointed out in #9, we are currently making the plot titles site ID - characteristicName. This makes it difficult to compare things between the Leaflet map and our static PNGs. It would be nicer if we could have the plot names be siteName - characteristicName using the info in p2_site_metdata.

We did not do this before the NMC conference because it would require regenerating all of the tar_visnetwork() screenshots used in the powerpoint slides to reflect the new dependency of p3_timeseries_plots_png on p2_site_metdata.