[ENH] Localizer and covariates

This PR introduces covariates fetching for the Localizer dataset.

Miscellaneous subject characteristics are available in two separate CSV files that can be downloaded on brainomics website (http://http://brainomics.cea.fr/localizer). They are now fetched together with images and put in the Bunch object that comes out of the downloader.

The covariates are not used yet in the examples, so there is no publicly-available use-case for it.

One caveat is that we need to ship the CSV files into the nilearn/tests/data directory in order to have the tests pass. Once again, this is a Nilearn issue that will be solved independently of this PR.