OKLab color gradients were partially implemented (#230)

in RadialGradient.kt and NPointLinearGradient.kt

Add OKLab variation to DemoNPointLinearGradient.kt

Mention OKLab (l, a, b) ranges and meaning.

orx-color/src/commonMain/kotlin/spaces/ColorOKLABa.kt

@@ -6,7 +6,10 @@ import org.openrndr.math.Vector4
 import kotlin.math.pow
 
 /**
- * Color in OKLab color space
+ * Color in OKLab color space.
+ * [l] = lightness: black (0.0) to white (1.0),
+ * [a] = red (-1.0) to green (1.0),
+ * [b] = yellow (-1.0) to blue (1.0).
  */
 data class ColorOKLABa(val l: Double, val a: Double, val b: Double, val alpha: Double = 1.0) :
     ConvertibleToColorRGBa,