revised thickinthehead release

We have revised the thickinthehead algorithm from the original published version. We removed upsampling to reduce memory issues for large image volumes, and replaced the estimated volume of middle cortical layer with an estimate of its surface area. We made these revisions to be less susceptible to deviations in voxel size from isometric 1mm^3 voxels for which thickinthehead was originally built (see issue #149).