Add support for creating sliding window of frames

[Zavster]

A better way of doing this...

var windowSize = 5;
var rowKeys = Range.ofLocalDates("2014-01-01", "2014-01-11");
var colKeys = Range.of(0, 5).map(i -> "Column-" + i);
var frame = DataFrame.ofDoubles(rowKeys, colKeys, value -> Math.random() * 10d);
IntStream.range(windowSize-1, frame.rowCount()).mapToObj(lastRow -> {
    var startRow = lastRow - windowSize;
    return frame.rows().select(row -> row.ordinal() <= lastRow && row.ordinal() > startRow);
}).forEach(window -> {
    ((DataFrame) window).out().print();
});