Bug fix: plots are now with 95 % CI assuming a t-distribution

Issue:
- As default, SPM plots 90 % Confidence Intervals assuming a Normal
distribution (which may not be accurate in small samples).
- For contrast with rank >1, no CI were added on the plots

Fix:
- The SwE toolbox now plots 95% CI and assume a t-distribution (which
is more accurate that a Normal distr.).
- We have removed the possibility to plot for contrasts with rank >1 as
the dof estimate for these contrasts may not be representative of the
dof estimates we would obtain if we were using as contrast (of rank =
1) each row of the contrasts (of rank >1) separately.
Bug fix: voxels with variance estimates = 0 are now masked

Issue:
- For some voxels having very (very) small values for the residuals,
the variance estimates of some visit category may be 0 (as it is
obtained from the squared of the residuals computed as 0 by Matlab).
These null variances may cause the failure of the toolbox.
Nevertheless, such a kind of voxels were always observed in the
background far away from the brain (so are of no interest).

Fix:
- The toolbox now automatically masks such a kind of voxels