construct based irt library and add item response function

app/lib/irt/item_response_function.py

@@ -0,0 +1,12 @@
+from models.three_parameter_logitistc import ThreeParameterLogistic
+
+class ItemResponseFunction():
+  def __init__(self, irt_model):
+    self.model_data = irt_model
+
+  def calculate(self, **kwargs):
+    if self.model_data.model == '3PL':
+      return ThreeParameterLogistic.new(self.model_data, kwargs).result
+    else:
+      # potentially error out
+      return None