add line function and its inverse

functions.py

@@ -2,6 +2,14 @@
 import numpy as np
 
 
+def line(x, m, c):
+    return m*x + c
+
+
+def inverse_line(x, m, c):
+    return (x-c)/m
+
+
 def exponential_function(x, a, b, c):
     return (a-c)*np.exp(-x/b)+c