Restructure 'model_type' in 'application.yaml'

[jeff1evesque]

We need to restructure our model_type from application.yaml:

    model_type:
        - svm
        - svr
        - bagc
        - bagr

To the following:

    model_type:
        classifiers:
            - svm
            - bagc
        regressors:
            - svr
            - bagr

This will means we'll need to refactor various aspects of our application, including bagger.py:

    classifiers = [list_model_type[3]]
    regressors = [list_model_type[4]]

Note: there are two instances of the above snippet in bagger.py.

Note: this issue was motivated by https://github.com/jeff1evesque/machine-learning/pull/3061#issuecomment-336729298.