use proper numpy eulers constant, better irt model abstraction

app/lib/irt/models/base.py

@@ -0,0 +1,14 @@
+import numpy as np
+
+class Base:
+
+  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 = np.exp
+    self.theta = kwargs['theta']
+
+  @classmethod
+  def ability_estimate(self) -> float:
+    return 0.0