Support general color maps

[MarcSkovMadsen]

It seems your apply_continuous_cmap is very tied to palettable.

I experienced this a friction as I'm used to using Colorcet. So I ended up creating some functionality to convert from colorcet to palettable in #262.

I'm not very well versed in colormaps but I would think that tying it to one provider is friction.

[kylebarron]

I hadn't previously used colormaps in python much, and so I figured the easiest approach was to nudge people towards a library with lots of colormaps.

To be clear, apply_continuous_cmap is only a helper for creating a numpy array that's of shape (n, 3) or (n, 4) and data type uint8 for passing into anything that accepts a color. Part of the primary goal in lonboard is to say "use the full range of python to do whatever you want that outputs a column of colors to render". So for example, I'm not familiar with colorcet but it looks like you can get matplotlib-compatible colormaps easily. So you can do

from matplotlib.cm import get_cmap

data = np.array(...)
colormap = get_cmap("cet_fire")
map_.get_fill_color = colormap(data, bytes=True)

(bytes=True makes it uint8 output 0-255 instead of float 0-1).

That said, it's probably good to update apply_continuous_cmap to support at least either any palettable colormap or any matplotlib colormap object (though for the matplotlib colormap it would literally just be an alias to call colormap(data, bytes=True).