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!
This seems like a strange default. Isn't it better to have the default be `np.eye(4)`? That is, the default is not to pre-apply any transformation?