relax SVN archive

Redesigned how diffusion tensor simulation structures are handled.

The design is now much cleaner and works with all Python versions.

Removed all the unused imports from specific_fns.model_free.main.

A number of private diffusion tensor objects and methods have switched to the single leading '_' format.

Improvements to the diffusion tensor set() method.

The parameters, errors and simulations are now properly differentiated and stored.

Converted the old diffusion tensor __setattr__() method into the set() method.

This is the only way in which diffusion tensor parameters, errors and simulations can be set.

Renamed the diffusion tensor data structure type() method to set_type().

This is because the type is stored as the 'type' object, clashing with the method name.

Created the diffusion tensor data structure type() method for setting the tensor type.

This is to remove the "cdp.diff_tensor.type = 'x'" code from the core of relax, as the structure is
now read only.

The new diffusion tensor fixed() method has been created to allow the fixed flag to be changed.

Fix for the initialisation of the diffusion tensor data structure, now that it is read-only.

The diffusion tensor data structure has been completely converted into a read-only structure.

The __setattr__() method now will always raise a RelaxError, and the diffusion tensor simulation
data structure objects __setitem__() method will raise the same error.