Structural changes

I must admit that pythons way to import from other folders still makes
my head ache... I think that all import paths must be relative the file
that is run as __main__, at least that seems to work. I named the folder
"tensors" since "math" is occupied, but both vectors, quaternions and
matrices are tensors, so it doesn't matter that much ;) Feel free to
change the structure if you think there are better ways.