add stage to whitelist/blacklist multiple namespaces

The NamespaceStrippingStage is useful but in many cases we either want to (a) remove multiple namespaces at once or (b) whitelist rather than blacklist. We should introduce a new NamespacesStrippingStage to do this, with a whitelisting property and a namespaces property taking a collection of namespaces to consider.

We should be able to refactor NamespaceStrippingStage so that the main logic is shared between the two stages.

There's an implementation of this in the ukf-mda project.