This is a method for the generic
unstack for the class "msr" of measures.
If x is a vector-valued measure, then
y <- unstack(x) is a list of scalar-valued measures
defined by the components of x.
The jth entry of the list, y[[j]], is equivalent to
the jth component of the vector measure x.
If x is a scalar-valued measure, then
the result is a list consisting of one entry, which is x.