Shifted the diffusion tensor structural object code to lib.structure.represent.diffusion_tensor.
The user function routes to pipe_control.structure.main.create_diff_tensor_pdb(), which pulls the
tensor info out of the data store, and then calls the diffusion_tensor() function of
lib.structure.represent.diffusion_tensor to create the representation, writes out a PDB file, and
finally adds the file to the data pipe's results file list.