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Fix for the eigenvalue and rotation matrix sorting for (Dxx, Dyy, Dzz, Dxy, Dxz, Dyz).

The eigenvalues were being converted to ints!

Fix for the ellipsoid eigenvalue and rotation matrix reordering for when 2 eigenvalues are the same.

This only affects the (Dxx, Dyy, Dzz, Dxy, Dxz, Dyz) parameter input into diffusion_tensor.init().

Added a new system test test_init_oblate_spheroid_as_ellipsoid().

Alphabetical ordering of methods, and shifting of the spheroid to ellipsoid checks into a separate method.

Renamed test_init_spheroid_as_ellipsoid() to test_init_prolate_spheroid_as_ellipsoid().

The print outs of the system test have been slightly improved.

The tensor is first twisted before rotating to PDB frame in the spheroid diffusion tensor system tests.

The twisting rotation was not being correctly performed.

Fix for the spheroid diffusion tensors for a recently introduced bug.

The oblate and prolate check was failing as the 'spheroid_type' variable was set to None after a
call to diffusion_tensor.init(). 'spheroid_type' is now automatically updated if Da is changed.

Added 6 system tests for the initialisation of the prolate spheroid diffusion tensor.

The hardcoded variables in get_spheroid() are now keyword args.

Some small cosmetic changes to the spheroid diffusion tensor system tests.