Adds a new labelstate.

        :param aminoAcidsLabels: Describes which amino acids can bear which
            labels. Possible keys are the amino acids in one letter code and
            'nTerm', 'cTerm'. Possible values are the modifications ids from
            :attr:`maspy.constants.aaModMass` as strings or a list of strings.
            An example for one expected label at the n-terminus and two expected
                labels at each Lysine:
                ``{'nTerm': 'u:188', 'K': ['u:188', 'u:188']}``
        :param excludingModifications: optional, A Dectionary that describes
            which modifications can prevent the addition of labels. Keys and
            values have to be the modifications ids from
            :attr:`maspy.constants.aaModMass`. The key specifies the
            modification that prevents the label modification specified by the
            value. For example for each modification 'u:1' that is present at an
            amino acid or terminus of a peptide the number of expected labels at
            this position is reduced by one: ``{'u:1':'u:188'}``

def addLabel(self, aminoAcidLabels, excludingModifications=None):
        """Adds a new labelstate.

        :param aminoAcidsLabels: Describes which amino acids can bear which
            labels. Possible keys are the amino acids in one letter code and
            'nTerm', 'cTerm'. Possible values are the modifications ids from
            :attr:`maspy.constants.aaModMass` as strings or a list of strings.
            An example for one expected label at the n-terminus and two expected
                labels at each Lysine:
                ``{'nTerm': 'u:188', 'K': ['u:188', 'u:188']}``
        :param excludingModifications: optional, A Dectionary that describes
            which modifications can prevent the addition of labels. Keys and
            values have to be the modifications ids from
            :attr:`maspy.constants.aaModMass`. The key specifies the
            modification that prevents the label modification specified by the
            value. For example for each modification 'u:1' that is present at an
            amino acid or terminus of a peptide the number of expected labels at
            this position is reduced by one: ``{'u:1':'u:188'}``

        """
        if excludingModifications is not None:
            self.excludingModifictions = True

        labelEntry = {'aminoAcidLabels': aminoAcidLabels,
                      'excludingModifications': excludingModifications
                      }
        self.labels[self._labelCounter] = labelEntry
        self._labelCounter += 1