Add routine to plot expressions from dseries objects.

Example:

>> ds1 = dseries(randn(100,3), dates('2000Q1'), {'x','y','z'});
>> ds2 = dseries(randn(100,3), dates('2000Q1'), {'x','y','z'});
>> dplot --expression 2*cumsum(x/y(-1)-1) --dseries toto --dseries noddy --range 2001Q1:2024Q1