Refactored DefaultFeature in two different implementation: DenseFeature and SparseFeature.
The intend is to have an efficient storage method in the common case where all attributes
are expected to be present (also known as "simple feature").


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK8@1595944 13f79535-47bb-0310-9956-ffa450edef68
Missing test file in previous commit.


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK8@1595945 13f79535-47bb-0310-9956-ffa450edef68