def add_external(self, fname, unit, binflag=False):
        if fname in self.external_fnames:
            print("BaseModel.add_external() warning: " +
                  "replacing existing filename {0}".format(fname))
            idx = self.external_fnames.index(fname)
            self.external_fnames.pop(idx)
            self.external_units.pop(idx)
            self.external_binflag.pop(idx)

        self.external_fnames.append(fname)
        self.external_units.append(unit)
        self.external_binflag.append(binflag)
        return
Assign an external array so that it will be listed as a DATA or
dataset/ETHPy150Open modflowpy/flopy/flopy/mbase.py/BaseModel.add_external