Shifted the specific_analyses.relax_disp.variables module into lib.dispersion.
This is both to minimise circular dependencies, as previously the specific_analyses.relax_disp
modules import from target_functions.relax_disp and vice-versa, and to allow the relax library
functions to have access to these variables. This follows from a similar change to the frame order
analysis in the frame_order_cleanup branch.