FIX: don't call _data_path on import

This fixes the problem that `mne.datasets.utils._data_path` is called for all datasets when importing mne, which had several unwanted side effects:

- Whenever importing mne, warnings about outdated datasets are generated.
- When using mne on a machine where the dataset is missing, the config file would always be overwritten with the default dataset location.

The second point doesn't seem like a big deal, but it creates a number of other problems. In my setup, I have all the datasets in `/tmp/data` on my workstation (for the nightly doc-build so the long paths don't show up on the website). Whenever using mne on a different machine (which doesn't have the sample data), the config file would be overwritten. This can be bad when e.g. running things on a cluster and several jobs try to read/write the config file at the same time (this just happened to me, resulting in a number of strange JSON Errors).