added helper function for featurizing a spec

stl/featurize.py

@@ -0,0 +1,13 @@
+from stl.fastboolean_eval import pointwise_sat
+
+def featurize_trace(phi, x):
+    params = {ap.name for ap in phi.params}
+    order = tuple(params)
+
+    def vec_to_dict(theta):
+        return {k: v for k, v in zip(order, theta)}
+
+    def eval_phi(theta):
+        return pointwise_sat(phi.set_params(vec_to_dict(theta)))(x, 0)
+
+    return eval_phi