Add endpoint to generate hierarchy of taxonomy from the backend

Strategy was to generate hierarchy directly once the item is requested through a GET. Therefore, to generate full hierarchy of the taxonomy, just need to iterate through all entries and request them through a GET on the API.

Closes #58

backend/metagenedb/api/catalog/views/taxonomy.py

+from rest_framework.response import Response
+
 from metagenedb.api.catalog.filters import TaxonomyFilter
 from metagenedb.apps.catalog.models import Taxonomy
 from metagenedb.apps.catalog.serializers import TaxonomySerializer
@@ -10,3 +12,9 @@ class TaxonomyViewSet(BulkViewSet):
     serializer_class = TaxonomySerializer
     lookup_field = 'tax_id'
     filterset_class = TaxonomyFilter
+
+    def retrieve(self, request, *args, **kwargs):
+        instance = self.get_object()
+        instance.build_parental_hierarchy()
+        serializer = self.get_serializer(instance)
+        return Response(serializer.data)