Move the "date string to ISO 8601" method to the NetCDF module, because it was used only
in the context of NetCDF file. This move will make easier to replace the JDK8 methods by
java.util.time formatter.


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK8@1587128 13f79535-47bb-0310-9956-ffa450edef68