add sigmoid function to basic models (#307)

fixes #306

Co-authored-by: Dominik Demuth <dominik.demuth@physik.tu-darmstadt.de>
Reviewed-on: #307

src/nmreval/models/basic.py

@@ -203,6 +203,31 @@ class Sinc:
         return c * np.sinc(((x-x0)/w)/np.pi)
 
 
+class Sigmoid:
+    type = 'Basic'
+    name = 'Sigmoid'
+    equation = 'C / [1 + exp(-a * (x - x_{0})] + y_{0}'
+    params = ['C', 'a', 'x_{0}', 'y_{0}']
+
+    @staticmethod
+    def func(x, c, a, x0, y0):
+        """
+        Sigmoid function
+
+        .. math::
+            y = C / [1 + exp(-a * (x - x_0))] + y_0
+
+        Args:
+            x (array_like): Input values
+            c (float): Prefactor
+            a (float): Steepness of the sigmoid
+            x0 (float): x position of the sigmoid's midpoint
+            y0 (float): y position of the sigmoid's midpoint
+
+        """
+        return c / (1 + np.exp(-a * (x - x0))) + y0
+
+
 class Sine:
     """
     Wavy sine function