Delete temp file aggressively.

[fukatani]

Delete temp file aggressively.
As refered in https://www.kaggle.com/tunguz/rgf-target-encoding-0-282-on-lb/code ,
sometimes user's environment has limited disk capacity, (ex. kaggle kernel has 1GB).

Currently, we clean temp file by @exit.register.
But some user need to delete temp file in run time.
I think it is natural that user expected release resources by garbage collector or __del__ method.

I mean,

rgf = RGFClassifier()
del rgf

But we should not expect to be called __del__ method every time.
For example, I confirmed test_parallel_gridsearch doesn't call __del__ method.

So we should also use @exit.register.

Another option to solve this problem is changing communication method between Python and C++.
For example, XGB uses ctypes.

[StrikerRUS]

For those who come here after closing the issue:

You can delete temp files associated with specific estimator by calling cleanup() method:

clf = RGFClassifier()
clf.fit(X, y)
clf.cleanup()

Note, that after this operation you will ought to call fit() method again to use the estimator.

Also you can delete all temp files of created estimators:

from rgf.utils import cleanup

cleanup()