Residual mean squares file at first and second level

Residual mean squares files is:
- 1st level: `stats/sigmasquareds.nii.gz`
- 2nd level: `stats/mean_random_effects_var1.nii.gz` + `stats/var_cope1.nii.gz` (in OLS `mean_random_effects_var1.nii.gz` exists and is equal to zero).