Add documentation

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Change development status to beta.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Unwrap hue when interpolating.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Added documentation to colormap.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Enhancements to colormap class.

 * __add__
 * reverse

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Added default colormaps.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Added the colorbar function.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>