Add support for spatially varying predictors

[nicholst]

(Low priority(!) item)

[nicholst]

How specify this...

X_files = ['/path/to/FirstSpatiallyVaryingRegressor.txt', 
           '/path/to/SecondSpatiallyVaryingRegressor.txt']

Each one of these files will have a list of N Nifti files inside.

And then user will need to know that contrasts will need that many more elements added at the end (relative to the non-spatially-varying design matrix X).

For now, no fancy masking options. I.e. if there are missing data in these spatially varying regressors, it is up to the user to modify the data masks to reflect this.