Redesigned the relax_disp.exp_type user function to be associated with spectrum IDs.

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 user function backend has been moved from specific_analyses.relax_disp.api to
specific_analyses.relax_disp.disp_data.  A few temporary code additions have been made to keep the
user function functional with the current dispersion code.
Fixes for the relaxation dispersion system tests for the relax_disp.exp_type changes.
The relaxation dispersion system tests requiring the compiled C modules are now skipped when not compiled.