Created the Diffusion_tensor.test_bug_21561_tensor_pdb_failure system test.
This is to catch bug #21561 (https://gna.org/bugs/?21561), as reported by Martin Ballaschk
(https://gna.org/users/mab).
This catches the failure of the structure.create_diff_tensor_pdb user function for non-spherical
diffusion tensors when no Monte Carlo simulations are present.
