add Smith-specific color ggplot palette

[beanumber]

https://drsimonj.svbtle.com/creating-corporate-colour-palettes-for-ggplot2

[beanumber]

More specifically:

• [ ] develop scale_color_smith_d(), which starts with Smith blue and then goes to Smith gold, but is also able to extrapolate n more perceptually distant colors.
• [x] develop scale_color_smith_c(), which is a sequential palette (a simple extension of scale_color_gradient() should do it)
• [x] develop scale_color_smith_c2(), which is a diverging palette that goes from Smith blue to white to Smith gold (scale_color_gradient2() should help)

Maybe a summer research project for @jcrouser 's students?

[beanumber]

Also,

• [ ] develop theme_smith() which perhaps changes the default font and adds a small Smith logo?