'Initialize plain array model.
See PanelModel'

def initialize(self, endog, exog, panel, time, xtnames, equation):

    names = equation.split('    ')
    self.endog_name = names[0]
    exog_names = names[1:]
    self.panel_name = xtnames[0]
    self.time_name = xtnames[1]
    novar = (exog.var(0) == 0)
    if (True in novar):
        cons_index = np.where((novar == 1))[0][0]
        exog_names.insert(cons_index, 'cons')
    self._cons_index = novar
    self.exog_names = exog_names
    self.endog = np.squeeze(np.asarray(endog))
    exog = np.asarray(exog)
    self.exog = exog
    self.panel = np.asarray(panel)
    self.time = np.asarray(time)
    self.paneluniq = np.unique(panel)
    self.timeuniq = np.unique(time)
