Bug 211839 String grouping unit type needs to support interval range.
Build ID: I20071101-2000 As Chart has supported, the String grouping unit type can be set interval range, the interval range means that the string group will base on row count distinction. For instance, let's say we have a,b,c,d,e,f,g , and set interval range to 2, the grouping result should be a,c,e,g. Now it is a requirement that DtE support the grouping type.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/odi/IQuery.java
/**
     * Defines a grouping criterion on an IQuery.
     */
    public static class GroupSpec
    {
        /**
         * No grouping interval unit specified.
         */
    	public static final int NO_INTERVAL = 0;
    	/**
         * Grouping interval unit is Year.
         */
    	public static final int YEAR_INTERVAL = 1;
    	
    	/**
         * Grouping interval unit is Month.
         */
    	public static final int MONTH_INTERVAL = 2;
    	
    	/**
         * Grouping interval unit is Quarter.
         */
    	public static final int QUARTER_INTERVAL = 3;
    	
    	/**
         * Grouping interval unit is Week.
         */
    	public static final int WEEK_INTERVAL = 4;