Permuted OLS: Correct the permutation scheme in haxby example.

The mean image per session and per condition is now considered.
Indeed, if no distinction is made, the observations are not exchangeable
across sessions and the permutation scheme is therefore wrong.
ENH: basic handling of NaNs in resampling

Right now there is no clever imputation, just filling with zeros
ENH: NaN in resampling: extrapolation

extrapolation before resampling avoids completing with zeros, which has
side effects in the resampling.
ENH: warn properly on NaNs in resample
BUG: support old scikit-learn versions
DOC: better warning messages
BUG: fix old versions of scikit-learn
Style
Remove code duplication
Permuted OLS: Address bthirion's comments.
Fetch files mock can fill csv data

For dataset fetching test, it was required to include valid CSV files. The
fetching API now allows the user to give the content of the CSV file as
recarray.
Get rid of oasis CSV file
Merge pull request #189 from VirgileFritsch/clever_permutations

[ENH] Permuted OLS: Correct the permutation scheme in haxby example.
Structure for RPBI with permuted_ols_alt_chunk.

GrowableSparseArrays' return.
WIP: RPBI with parietal-python wards construction.
WIP: RPBI computation ends but h0 has a weird shape (lots of 0 and 'max').
forgotten files.
Investigate parcellations mutual info.
Test RPBI with random parcels (Wards on noise).
Group by sessions in example.