Implement Stack class

Add a class to represent a collection of stacked processes in a hist()-histogram. The class should store

the way the total uncertainty is computed (e.g. s=\sqrt{\sum_i{w_i}} for data/asimov, or s=\sqrt{\sum_i{w_i^2}} for mc)
the colors,
the type step/stepfilled vs points,
the style (dashed, markersize, ...)
etc.

When adding a stack, it should be possible to change the properties of this process only. For example, it should be possible to add a step process to an stepfilled stack.

It would be conveniet to define subclasses McStack and DataStack which the following defaults:

DataStack: points, s=\sqrt{\sum_i{w_i}}
McStack: stepfilled, s=\sqrt{\sum_i{w_i^2}}