Retrofit the AbstractSingleCRS subclass in its AbstractCRS parent class in an effort to simplify our class hierarchy.
This is based on Geotk experience where it appears that the distinction between SingleCRS and "general CRS" is useful
in interfaces, but not necessary in the implementation classes.


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK7@1555398 13f79535-47bb-0310-9956-ffa450edef68