use proper numpy eulers constant, better irt model abstraction

app/lib/irt/models/rasch.py

@@ -1,14 +1,8 @@
-import numpy as np
-
 from girth import ability_mle
 
-class Rasch:
+from lib.irt.models.base import *
 
-  def __init__(self, model_params, kwargs):
-    self.model_params = model_params
-    self.b_param = kwargs['b_param']
-    self.e = 2.71828
-    self.theta = kwargs['theta']
+class Rasch(Base):
 
   def result(self):
     return 0.0