train_model: problem with model_id in multiple nnetar method

[VitoFanelli]

The model_id in nnetar method must be "nnetar" (not "nnetar1" or "nnetar2"). This imply one nnetar model at a time and not multiples in train_model function. In case of model_id different from "nnetar" train_model functions return an error, otherwise it works correctly.

The file belows contains an example.
nnetar_problem_tsstudio.txt