RMSSS(): accept vectors as input data

The current RMSSS() requires the input exp and obs to have at least two dimensions, 'sdate' and 'member'. It is because s2dv is tailored for seasonal forecast so ensemble is considered. However, if someone wants to use RMSSS() for vector (i.e., 1D time series data), it should be work too.

In the next release, modify the function to accept 1D data as input.