check the prec parameter is defined

classical/systems.py

@@ -130,6 +130,7 @@ class Morse(PhysicalSystem):
             isclose = np.isclose
             times = np.array(times, dtype=np.float)
         else:
+            assert self.prec is not None, '"prec" parameter not defined'
             from sympy import exp, sqrt, sign
             from sympy import acos as arccos
             from sympy import acosh as arccosh