affine can be None for fvwm='fast'

Minor modification to enable not specifying an affine
DOC: document that we are not using affine for fwhm='fast'
Enhance error message if no folder is writable
Merge pull request #258 from GaelVaroquaux/misc_fast_smooth_array

Affine can be None for fvwm='fast'
Merge branch 'master' into sparse-models
renamed n_voxels -> n_features
renamed _intercepted_prox_tvl1 -> _prox_tvl1_with_intercept
renamed logistic_grad -> logistic_loss_grad + no longer importing div as divergence