fix matplolib import error

src/4_sommerfeld_model_solutions.md

@@ -130,6 +130,8 @@ CV = (dEplus - dEmin) / (2*dT);
 1.
 
 ```python
+import numpy as np
+import matplotlib.pyplot as plt
 x = np.linspace(-1, 1, 100)
 fig, ax = plt.subplots(figsize=(7, 5))
 ax.plot(x, x, 'b')