Adapt velocity layer for an easier use with a ColormapControl

[HaudinFlorence]

Adapt velocity layer re-building the color_scale list from a linear colormap of Branca. This enables to easily use a ColormapControl with this layer using the same colormap in the layer and in the control.

[davidbrochart]

Thanks @HaudinFlorence! It could be good to update the documentation also?

[HaudinFlorence]

Thanks @HaudinFlorence! It could be good to update the documentation also?

The docstrings have been updated.