Compute a set of colors diverging
from a neutral center (grey or white, without color) to two
different extreme colors (blue and red by default). For the
diverging HSL colors, again two hues :h are needed, a maximal
saturation ':s' and two lightnesses ':l'.  The colors are then created by
an interpolation between the full color hsl1,
a neutral color hsl and the other full color hsl2.

Arguments:
numcolors: Number of colors to be produced in this palette.

Optional Arguments:
:h-start (keyword) starting hue (default 260)
:h-end (keyword) ending hue (default 0)
:s (keyword) saturation. 0.0 - 100.0 (default 80.0)
:l-start (keyword) starting lightness. 0.0 - 100.0. (default 30.0)
:l-end (keyword) ending lightness. 0.0 - 100.0. (default 90.0)
:power (keyword) control parameter determining how saturation and lightness should
be increased (1 = linear, 2 = quadratic, etc.) (default 1.5)

Create heat palette in HSL space. By default, it goes from a red to
  a yellow hue, while simultaneously going to lighter colors (i.e.,
  increasing lightness) and reducing the amount of color (i.e.,
  decreasing saturation).

Arguments:
numcolors: Number of colors to be produced in this palette.

Optional Arguments:
:h-start (keyword) starting hue (default 260)
:h-end (keyword) ending hue (default 260)
:s-start (keyword) starting saturation. 0.0 - 100.0 (default 80.0)
:l-start (keyword) starting lightness. 0.0 - 100.0. (default 30.0)
:s-end (keyword) ending saturation. 0.0 - 100.0 (default 0.0)
:l-end (keyword) ending lightness. 0.0 - 100.0. (default 90.0)
:power-saturation (keyword) control parameter determining how saturation should increase
:power-lightness (keyword) control parameter determining how lightness should increase
be increased (1 = linear, 2 = quadratic, etc.)

Return n evenly spaced points along the range start - end (inclusive).

Computes a rainbow of colors (qualitative palette) defined by
different hues given a single value of each saturation and lightness.

Arguments:
numcolors: Number of colors to be produced in this palette.

Optional Arguments:
:s (keyword) saturation. 0.0 - 100.0 (default 50.0)
:l (keyword) lightness. 0.0 - 100.0. (default 70.0)
:start (keyword) hue where rainbow starts. 0 - 360 (default 0)
:end (keyword) hue where rainbow ends. 0 - 360 (default: (* 360 (/ (- numcolors 1) numcolors)))

Creates a sequential palette starting at the full color
 (h :s-start :l-start) through to a light color (h :s-end :l-end) by
interpolation.

Arguments:
numcolors: Number of colors to be produced in this palette.

Optional Arguments:
:h (keyword) starting hue (default 260)
:s-start (keyword) starting saturation. 0.0 - 100.0 (default 80.0)
:l-start (keyword) starting lightness. 0.0 - 100.0. (default 30.0)
:s-end (keyword) ending saturation. 0.0 - 100.0 (default 0.0)
:l-end (keyword) ending lightness. 0.0 - 100.0. (default 90.0)
:power (keyword) control parameter determining how saturation and lightness should
be increased (1 = linear, 2 = quadratic, etc.)

The 'terrain_hcl' palette simply calls 'heat_hcl' with
different parameters, providing suitable terrain colors.