Returns a computed relevance score for the given node. The score is the sum of all relevance scores provided by Lucene for the node and its descendants. In general, the score will be a number between 0.0 and 1.0 if the query had $node as context. If the query targeted multiple descendants of $node (e.g. 'title' and 'author' within a 'book'), the score will be the sum of all sub-scores and may thus be greater than 1.