removed "findEntities", completed use of "getEntities" & removed "children" methods

Undone findEntities and reintroduced "findSections" & "findSources" instead

Implemented use of "getEntities" in last missing instance, i.e. "EntityWithSources".
Also: added "getSource(size_t index)" to "IEntityWithSources" to require all derived classes to implement it for use with "getEntities".

Removed the "children" methods in "Sections" & "Sources" that had been added when implementing (the now also removed) "findEntities"