Connect rasters

[jessicaguo]

This PR does the following:

• reprocesses amp and vel data so that 0's become NAs
• connect 3 out of 4 rasters to the app
• specifies color palette so the values a) match the previous app and b) stay the same over time

Could use improvement:

• I played around in notes/plot_rasters_leaflet.R with native legends. While they looked alright for the most part (not sure how to label the colorFactor for Stratification), I couldn't easily figure out how to make them play nicely with map_proxy. What do you think would be easier/more visually pleasing, a legend on the map itself or separate from the map?
• Similarly, I used an inelegant set of nested conditionals to remove the coastline, and it lags. Can you suggest a better way to integrate click box input for coastline?

[keatonwilson]

@jessicaguo Check out the commits - I got legends working with proxy. A few necessary steps:

1. adding the legend to the base map
2. adding legends to each conditional chunk (I only filled in the one that is built - the rest I'll leave up to you)
3. adding clearControls functions to the top of the proxy builds - this will wipe out the existing legend before adding. It prevents a bug where if you turned coastline off and on, the ice/land legend would keep duplicating onto the map.

I'm not sure it's actually the nested conditionals that are causing the slow response time - it may be the volume of data that we're starting to process and stack. Sophie mentioned that downsampling might be a reasonable alternative, so we have some options.

My vote is to get through initial buildout of things recognizing that things are getting slow, and then spend some time at the end re-tooling and speeding things up. What do you think?

[jessicaguo]

Nicely done! I admit to beating a hasty retreat when I saw the neverending legends.

I agree about the order of operations, just not familiar with the ways to speed things up. But I trust you'll get us there!