Created the specific_analyses.relax_disp.disp_data.loop_exp*() functions.

This follows from http://thread.gmane.org/gmane.science.nmr.relax.devel/4530, the thread about
supporting multiple data types such as SQ+MQ data simultaneously.

The methods added are loop_exp(), loop_exp_frq(), loop_exp_frq_point() and
loop_exp_frq_point_time().