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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def forecast(self,parensemble=None):
 """for the enkf formulation, this simply moves the ensemble forward by running the model once for each realization"""

        if parensemble is None:
            parensemble = self.parensemble
        self.logger.log("evaluating ensemble")
        failed_runs, obsensemble = self._calc_obs(parensemble)


        if failed_runs is not None:
            self.logger.warn("dropping failed realizations")
            parensemble.loc[failed_runs, :] = np.NaN
            parensemble = parensemble.dropna()
            obsensemble.loc[failed_runs, :] = np.NaN
            obsensemble = obsensemble.dropna()
        self.logger.log("evaluating ensemble")

        return obsensemble