Initial support of Attribute in Feature, for now only in the case of [0...1] cardinality.
This cover the vast majority of cases and is handled in a special way for lower memory consumption.
We still need to implement the multi-values case (cardinality [0...n]) and add test and documentation.


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK8@1592633 13f79535-47bb-0310-9956-ffa450edef68