A new `foreach` pipeline step has been added to the :ref:`beta-plugin`.  This step allows
a pipeline's structure to vary dynamically using a list of items, e.g. to summarize an
event loss table against many different region layers. 

The `foreach` expression must yield a constant value, but it can specify either a list, a struct, 
or a single item. If a struct is returned then each attribute value in the struct becomes an `$item`
that is looped over (the attribute name is used as the `$index` in the loop). (GL#1469, GL#1716, GL#1726)
