Add trend to monthly chlorophyll plot

[BBeltz1]

Kim would like us to try adding trends to the monthly chlorophyll plot (ecodata::plot_chl_pp(varName = "chl", plottype = "monthly")).

[andybeet]

This is easy to do. However, i think earlier this week we decided to wait until after this SOE cycle before applying the "short term trend methods" to data sets < 30 data points

[khyde]

Yes, I think we were going to wait on applying it to the whole time series, but we were going to apply it to the last 10 years.