Added fitmonitor import

The `ba_fitmonitor` was called without having been imported. This PR resolves this.

Examples/fit55_SpecularIntro/FitSpecularBasics.py

@@ -8,6 +8,7 @@ Reference data was generated with GENX for ti layers' thicknesses equal to 3 nm
 
 import numpy as np
 import bornagain as ba
+from bornagain import ba_fitmonitor
 from matplotlib import pyplot as plt
 from os import path