Blurb::
Assign each member to a layer, based on domination the rank based on layers
Description::
The ``fitness_type``: ``layer_rank`` has been specifically designed to
avoid problems with aggregating and scaling objective function values
and transforming them into a single objective.

The ``layer_rank`` fitness assessor works by assigning all
non-dominated designs a layer of 0, then from what remains, assigning
all the non-dominated a layer of 1, and so on until all designs have
been assigned a layer. The values are negated to follow the
higher-is-better fitness convention.

Use of the ``below_limit`` selector with the ``layer_rank`` fitness
assessor has the effect of keeping all those designs whose layer is
below a certain threshold again subject to the shrinkage limit.
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