add vanderpol example

Author: StephenSmith25

examples/vanderpol_me.py

@@ -0,0 +1,32 @@
+from pythonfmu3 import Fmi3Causality, ModelExchange, Fmi3Variability, Fmi3SlaveBase, Fmi3Status, Float64, Fmi3Initial, Unit, Float64Type, Fmi3StepResult
+
+from typing import List
+
+class VanDerPol(Fmi3SlaveBase, ModelExchange):
+
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+
+        self.author = "Stephen Smith"
+        self.description = "Van Der Pol oscillator problem for model exchange FMUs"
+
+        self.time = 0.0
+        self.mu = 1.0
+        self.x0 = 2
+        self.x1 = 0
+        self.derx0 = 0.0
+        self.derx1 = 0.0
+
+        self.register_variable(Float64("time", causality=Fmi3Causality.independent, variability=Fmi3Variability.continuous))
+        self.register_variable(Float64("x0", causality=Fmi3Causality.output, start=2, variability=Fmi3Variability.continuous, initial=Fmi3Initial.exact))
+        self.register_variable(Float64("x1", causality=Fmi3Causality.output, start=0, variability=Fmi3Variability.continuous, initial=Fmi3Initial.exact))
+        self.register_variable(Float64("derx0", causality=Fmi3Causality.local, variability=Fmi3Variability.continuous, derivative=1))
+        self.register_variable(Float64("derx1", causality=Fmi3Causality.local, variability=Fmi3Variability.continuous, derivative=2))
+        self.register_variable(Float64("mu", causality=Fmi3Causality.parameter, variability=Fmi3Variability.fixed))
+
+
+    def get_continuous_state_derivatives(self) -> List[float]:
+        self.derx0 = self.x1
+        self.derx1 = self.mu * ((1 - self.x0**2) * self.x1) - self.x0
+        return [self.derx0, self.derx1]
+