Added method dynTimeIndex::mpower -> lead-lag operators à la G-M ;-).

This method can be used to apply the lead and lag methods an arbitrary number of times to a dynSeries object. For instance, if ts is a dynSeries object, and if we define

>> B = dynTimeIndex()-1;
>> F = dynTimeIndex()+1;

B and F can be used as lag and lead operators and the following syntax:

>> us = ts(F^2);

is equivalent to

>> us = ts.lead(2)

or

>> us = ts.lead.lead