Gh-3210: Fix GafferPopVertex.properties() output

[p29876]

Set a proper ID for GafferPopVertexProperties so that equals and hashCode work as expected simplify GafferPopVertex.properties() method add unit test to catch error

Related issue

• Resolve #3210

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.85%. Comparing base (759531c) to head (bcf5dce).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3211 +/- ## ============================================= - Coverage 66.85% 66.85% -0.01% Complexity 2555 2555 ============================================= Files 912 912 Lines 29284 29276 -8 Branches 3268 3265 -3 ============================================= - Hits 19578 19572 -6 + Misses 8259 8258 -1 + Partials 1447 1446 -1 ```

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