Improve scatter plot

[forman]

Expected behavior

Operation plot_scatter should be applicable to N-D datasets. This is a feature request by Paolo Cipollini.
It should also allow for creating 2D Histogram and Hexbin plots.

Actual behavior

Operation plot_scatter is currently restricted to 1-D, which means users have to set indexers so that N-D variables are reduced to 1-D.
The only plot type is "points", which is not useful for large number of points as points are drawn on to of each other.

Steps to reproduce the problem

1. Open 3-D data, e.g. Clouds CCI products
2. Run operation plot_scatter

Specifications

Cate 2.0.dev1 - 2.0.dev19

[forman]

Fixed in dev20