Return LayerArray by resource-managing type (was ACTS-83)

Original author Christian Gumpert @cgumpert

The LayerArray return by the ILayerArrayCreator.h is returned as raw pointer. We should be consistent and use resource managing objects for all kinds of factory methods. The usage of a {{std::unique_ptr}} might be appropriate in this place.

Edited Mar 27, 2019 by Moritz Kiehn

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