Created a number of system tests to check the diffusion tensor PDB representation.
This is to prevent bugs such as #22502 (https://gna.org/bugs/?22502) from ever reappearing. The PDB
file contents are hardcoded into the tests and checked. The tests include
Structure.test_create_diff_tensor_pdb_ellipsoid, Structure.test_create_diff_tensor_pdb_oblate,
Structure.test_create_diff_tensor_pdb_prolate, and Structure.test_create_diff_tensor_pdb_sphere.
