Row indices are used to select a subset of the original time period.
The start and epoch slots of the time series are
adjusted accordingly.
A warning is issued if an irregular integer sequence is used to
extract rows, e.g., x[c(1,2,4),], which could destroy the
structure of the time series (freq).
Column indices work as usual when indexing matrices,
so may select units by name, position or a vector of booleans.
When subsetting columns, population fractions are recomputed if and
only if x is no multinomialTS and already contains
population fractions.
NA indices are not supported, negative indices are.
Note that a [<- method (i.e., subassignment) is not implemented.