Fixed typo (cosmetic).
Added condition on dynSeries::lag method (input must be positive).
Added dynSeries::lead method.
Added new class (dynTimeIndex).
Introduce new syntax for shifting backward or forward a dynSeries object.

Suppose that ts is a dynSeries object and let

t = dynTimeIndex();

Then

us = ts.lag();  <=>  us = ts(t-1);

us = ts.lag(2);  <=>  us = ts(t-2);

us = ts.lead();  <=>  us = ts(t+1);

Added corresponding unitary test.