Makes a blank version of stim. If a type is not given, returned as same type as current stim.
        If a column stim, will fill in blanks with ``fill``

def blank_stim(self,type=None,fill=0):
        '''Makes a blank version of stim. If a type is not given, returned as same type as current stim.
        If a column stim, will fill in blanks with ``fill``'''
        blank = copy.copy(self)
        blank.name = 'Blank'
        if type==None:
            type = self.type()
        if type=="column":
            num_reps = self.reps
            if num_reps==None:
                if self.type()=="column":
                    self.read_file()
                    num_reps = len(self.column)
                else:
                    nl.notify('Error: requested to return a blank column, but I can\'t figure out how many reps to make it!',level=nl.level.error)
            blank.column = [fill]*num_reps
            return blank
        if type=="times":
            blank.times = []
            return blank