More report enhancements

[bmaitner]

• [x] combine NDVI plots
• [x] Move color ramps on NDVI plots to bottom
• [ ] Make it so the legend is only shown when needed (if possible)
• [x] Re-organize layer tickboxes (base layers, parks, NDVI stuff, fire stuff)
• [x] Remove NDWI (comment out)
• [ ] slider for hiding the ndvi layer(s) (if possible)
• [ ] graphs smaller, text larger
• [x] add park-specific age over time plot. y-axis is cells, x-axis is date, color = age (time since fire).
• [ ] invasives summarized as hex bins (depending on the number we want to include)

[bmaitner]

Re: color ramps on the bottom: this doesn't seem possible. Per the documentation, the only options are along the right or left sides (topright, bottomright, topleft, bottomleft). Since we added a third color legend, I moved the layer control to the left side of the plot.