Pre fetch openml data for pytest

[franchuterivera]

It prefetches the openml data on a need basis. With pytest -s option one would see:

test/test_data/test_validation.py::test_data_validation_for_classification[True-2] DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
DEBUG:openml.datasets.dataset:Data pickle file already exists and is up to date.
Pre-Fetch of 14 tasks took 2.0051114559173584 seconds...

Addresses #109