add new colormap trait

- get_color(h) takes a floating point value between 0.0 and 1.0 and
  returns a color corresponding to this value
- get_color_normalized(h, min, max) does the same but sets the bounds to
  min and max instead of 0.0 and 1.0
- the floating point type is f32 by default but can be chosen
  arbitratily as long as it complies with the trait defined in the
  num_traits package

Author: jonaspleyer

plotters/src/style/colors/colormaps.rs

@@ -0,0 +1,10 @@
+pub trait ColorScale<ColorType: crate::prelude::Color, FloatType=f32>
+where
+    FloatType: Float,
+{
+    fn get_color(&self, h: FloatType) -> ColorType {
+        self.get_color_normalized(h, FloatType::zero(), FloatType::one())
+    }
+
+    fn get_color_normalized(&self, h: FloatType, min: FloatType, max: FloatType) -> ColorType;
+}