Added double-double arithmetic support as an internal class.
For now we plan to use it only for matrix multiplications and inversions, where
(in the particular case of SIS) accuracy is more critical than performance.


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK7@1526663 13f79535-47bb-0310-9956-ffa450edef68