Clarify entityID types in Merger class

[eileenmcnaughton]

Overview

Clarify entityID types in Merger class

Before

As a carry over from when the getTree function was shared there is a lack of clarity about the type of $entityID - this is because in the shared usage it could also be NULL

After

The IDs are coming from the getRowsElementsAndInfo so I added typing to where that function receives the values & cast the values in all the places that call it (except the tests which will tell us if they are not already the right type...)

Technical Details

Comments

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[civibot]

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