Changed the behaviour of the sort method...

... and added a new method.

sort_ sorts a dates object in place (without a copy).
sort  first make a copy and then sorts the copy.

The first routine, with in place modification, is more efficient because
we do not have to create a new object.

Example
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If o is a dates object with more than one element, then

o.sort()

returns a sorted version of o without modifying o, while

o.sort_()

sorts the elements in o.