A Sample or outer LayerStack may contain one or several instances
of class LayerStack.
Each instance is created using the constructor call
stack = ba.LayerStack(n_periods)
or
stack = ba.LayerStack() # by default n_periods = 1
The stack is created empty. To populate it with layers or other stacks, call
stack.addLayer(layer)
stack.addStack(sub_stack)
The numper of repetitions can be changed with
stack.setNumberOfPeriods(n_periods)
or returned with
stack.numberOfPeriods()