Add docstrings in Nordigen functions

handler/sinks/nordigen.py

@@ -45,6 +45,10 @@ class Nordigen(object):
     def get_institutions(self, country, filter_name=None):
         """
         Get a list of supported institutions.
+        :param country: country to query
+        :param filter_name: return only results with this in the name
+        :return: list of institutions
+        :rtype: list
         """
         if not len(country) == 2:
             return False