input: Write a function to classify species using a given species list.
  assert classifier.classify_species('cat') == 'mammal'
  assert classifier.classify_species('sparrow') == 'bird'
  assert classifier.classify_species('goldfish') == 'fish'
output: class SpeciesClassifier:
      def __init__(self):
          self.species_dict = {
              'cat': 'mammal',
              'dog': 'mammal',
              'sparrow': 'bird',
              'eagle': 'bird',
              'goldfish': 'fish',
              'shark': 'fish'
          }
  
      def classify_species(self, species):
          return self.species_dict.get(species, 'unknown')
  
  classifier = SpeciesClassifier()