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

app/lib/irt/models/three_parameter_logistic.py

@@ -0,0 +1,13 @@
+class ThreeParameterLogistic:
+  def __init__(self, model_params, kwargs):
+    self.model_params = model_params
+    # check if exists, if not error out
+    self.b_param = kwargs['b_param']
+    self.e = 2.71828
+    self.theta = kwargs['theta']
+
+  def result(self):
+    a = self.model_params.a
+    c = self.model_params.c
+
+    return c + (1 - c) * (1 / (1 + e**(-a * (self.theta - self.b_param))))