Warn user if step_target is not set correctly or separate it from final time step

[SarahAlidoost]

The model.run() runs while self.step_count < self.step_target. Imagine, step_target=100 but we want only to run 10 time steps. Currently, this is possible if before model.run() , the variable step_target is set to a smaller value. e.g. the cod below is working:

model = dgl_ptm.PovertyTrapModel(model_identifier='test2')
model.set_model_parameters()  # Default step_target is 100
model.initialize_model()
model.step_target = 10
model.run()

The variable step_target is used in _initialize_model_theta to calculate the array of modelTheta. Before performing run(), If step_target is changed to a value greater than was used initially in _initialize_model_theta e.g. step_target=200, the model run fails. e.g. the code below is not working:

model = dgl_ptm.PovertyTrapModel(model_identifier='test2')
model.set_model_parameters()  # Default step_target is 100
model.initialize_model()
model.step_target = 200
model.run()