Add model_filename and keep_tempfiles attrs to CRF instance

[HarryCaveMan]

Add model_filename and keep_tempfiles attr to self to comply with sklearn clone api which requires all __init__ args be instance attributes.

This is imperative for functionalities in sklearn which use parameterized cloning of estimators. The API requires all __init__ arguments to also be instance attributes or an AttributeError` will be raised: https://github.com/scikit-learn/scikit-learn/blob/7f9bad99d6e0a3e8ddf92a7e5561245224dab102/sklearn/base.py#L194C16-L194C16