Removed the possibility to extract subsamples from a dseries object with a vector of integers.

If ts is a dates object with 100 observations (ts.nobs=100), the
following syntax for selecting the last 90 observations *is not legal*:

ts(11:end)

ts has to be indexed instead with dates objects, as in the following example:

ts(d1:d2)

where d1 and d2 are dates objects.  To select the 90 last observations,
if the last date is unknown, the following syntax can be used:

ts(ts.dates(11:end))