update exec test

See if this passes now

tests/exec_test/check_model.py

-from cpp_sisso import Model
+from cpp_sisso import ModelRegressor
 from pathlib import Path
 
 import numpy as np
 
-model = Model(
+model = ModelRegressor(
     str("models/train_dim_2_model_0.dat"), str("models/test_dim_2_model_0.dat")
 )