Improved In Memory Dataflow so that joins and filters would work.

CHANGELOG.md

CRM/Dataprocessor/Utils/DataSourceFields.php

Civi/DataProcessor/DataFlow/InMemoryDataFlow.php

...aProcessor/DataFlow/InMemoryDataFlow/CallbackDataFlow.php

...taProcessor/DataFlow/InMemoryDataFlow/FilterInterface.php

.../DataProcessor/DataFlow/InMemoryDataFlow/SimpleFilter.php

Civi/DataProcessor/DataFlow/MultipleDataFlows/SimpleJoin.php

Civi/DataProcessor/DataSpecification/DataSpecification.php

...ataProcessor/FilterHandler/AbstractFieldFilterHandler.php

Civi/DataProcessor/FilterHandler/SimpleSqlFilter.php

Civi/DataProcessor/Source/AbstractSource.php