Added the NestedRelation#290

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alexberezinsky wants to merge 5 commits intospotorm:masterfrom

alexberezinsky:nested_relation

Commits on May 28, 2019

Added the NestedRelation
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Commits on Jun 19, 2019

Sample test
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Commits on Jun 25, 2019

A
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A
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Commits on Jun 26, 2019

Removed the nestedRelation() method from Mapper and from MapperInterface.
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