add publication class template

input/pub.py

+class Publication:
+    #_registry = []
+    #_citations = []
+    #_references = []
+    
+    def __init__(self, title, publication_date, contributors, doi_url, 
+                 subjects, num_citations):
+        #self._registry.append(self)
+        self.title = title
+        self.publication_date = publication_date
+        self.contributors = contributors
+        self.doi_url = doi_url
+        self.subjects = subjects
+        self.num_citations = num_citations
+        self.num_references = num_references
+	self._citations = []
+	self._references = []
+
+class Citation:
+    def __init__(self, title, journal, contributors, doi_url):
+        self.title = title
+        self.journal = journal
+        self.contributors = contributors
+        self.doi_url = doi_url
+
+class References:
+    def __init__(self, title, journal, contributors, doi_url):
+        self.title = title
+        self.journal = journal
+        self.contributors = contributors
+        self.doi_url = doi_url
+