Implement equals() and hashCode() in all QueryElement implementations
|Reported by:||Nicklas Nordborg||Owned by:||Nicklas Nordborg|
This is a feature that would be useful since we then don't have to compare Expressions using their string representations. The comparison is needed in the BFS exporter when used from the external program executor to make it possible to check if a given expression has already been added to the export. Right now, we check this by calling Expression.toString() and comparing the result, but it would be better if expressions implemented equals() instead.