Initialize the tallyable objects.

        Makes sure a Trace object exists for each variable and then initialize
        the Traces.

        :Parameters:
        funs_to_tally : dict
          Name- function pairs.
        length : int
          The expected length of the chain. Some database may need the argument
          to preallocate memory.

def _initialize(self, funs_to_tally, length=None):
        """Initialize the tallyable objects.

        Makes sure a Trace object exists for each variable and then initialize
        the Traces.

        :Parameters:
        funs_to_tally : dict
          Name- function pairs.
        length : int
          The expected length of the chain. Some database may need the argument
          to preallocate memory.
        """

        for name, fun in six.iteritems(funs_to_tally):
            if name not in self._traces:
                self._traces[
                    name] = self.__Trace__(
                        name=name,
                        getfunc=fun,
                        db=self)

            self._traces[name]._initialize(self.chains, length)

        self.trace_names.append(list(funs_to_tally.keys()))

        self.chains += 1