fix coverage metric aggregation
Fix aggregation with no predictions (#401)

- Add userRow() and finalRow() convenience methods to help
  build user/final result rows more easily
- Return null/NA when no users are scored in a metric

Conflicts:
	lenskit-eval/src/main/java/org/grouplens/lenskit/eval/metrics/predict/EntropyPredictMetric.java
	lenskit-eval/src/main/java/org/grouplens/lenskit/eval/metrics/predict/HLUtilityPredictMetric.java
	lenskit-eval/src/main/java/org/grouplens/lenskit/eval/metrics/predict/MAEPredictMetric.java
	lenskit-eval/src/main/java/org/grouplens/lenskit/eval/metrics/predict/NDCGPredictMetric.java
	lenskit-eval/src/main/java/org/grouplens/lenskit/eval/metrics/predict/RMSEPredictMetric.java
	lenskit-eval/src/main/java/org/grouplens/lenskit/eval/metrics/topn/NDCGTopNMetric.java
	lenskit-eval/src/main/java/org/grouplens/lenskit/eval/metrics/topn/TopNLengthMetric.java