Optimization/IOMixin: Fix set_timeseries for subslices of times

The stretch_values function would only work if the subslice Timeseries extended to the end of the existing io.times_sec. In other words, it would only ever prepend with NaNs, and never append them (contrary to what the nearby comment stated).

This commit makes sure that we can call set_timeseries with slices that neither start nor end with the first and last value of io.times_sec. This can be useful when e.g. importing 1000 times with CSVMixin, but overriding times() to run timestep 100-150.