docs: def last(self):
          """Last time step available.
  
          Example:
              >>> sdat = StagyyData('path/to/run')
              >>> assert(sdat.steps.last is sdat.steps[-1])
          """
          if self._last is UNDETERMINED:
              # not necessarily the last one...
              self._last = self.sdat.tseries.index[-1]
          return self[self._last]