Write the save functions to save the trained models into files

[kan-qi]

The goal is to save the trained models into the files after we evaluate them.

1. we train the models here:

2. in this function there is a iterative process to save the models (parameters) into files.

this m_save is a S3 generic function, which will call the specific m_save function of each model, for example,

3. some external links to save R objects into files: https://www.r-bloggers.com/a-better-way-of-saving-and-loading-objects-in-r/