Shifted the matrix output of the structure.atomic_fluctuations user function into lib.plotting.text.
The new lib.plotting.text module will be used by the relax library plotting API to output data into
plain text format. The current correlation_matrix() function, which has been added to the API
correlation_matrix() function dictionary, simply has the file writing code of the
structure.atomic_fluctuations user function. This significantly simplifies the user function.