Add design pattern for migratory cells

[cmungall]

Motivation:

There is currently no relationship between lung migratory cell and lung. This is ontologically correct - it can migrate in and out of the lungs. But there is clearly a relationship that should be articulated. Adding this would facilitate queries that are broader than strict ontological ones, e.g. making a lung subset of CL

The relationship could be temporal (dare I say, part of at some times...), or functional

[github-actions[bot]]

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