Enhance SLR with applying series of transformations

So, now you can start with rigid and then do similarity and then affine like you would do in ANTs. You can just give the matrix of the previous optimization. @omarocegueda this implements what we discussed before. It may give us some ideas for the linear image-based registration. GGT!
No eye(4) would be a very bad strategy. The default idea in registration is that you first find the center of mass of the two objects that you want to register and then you move them to the origin. In that way you make sure that your rotations during the registration will make sense. Is that clear?