Prevent multiple values conflict in FindExpressionRoot init

[meteospike]

In module loki.expression.expr_visitors the initialization of class FindExpressionRoot does not check for unique keyword argument presence, which could lead to a multiple values conflict since this argument is statically set in the super.__init__ call.

This could be easily fixed, adding the line:
kwargs.pop('unique', None)

[reuterbal]

Thanks a lot for reporting, this is a genuine bug and I agree on the fix. I'll implement it now