Created the new specific_analyses.relax_disp.checks module.

This contains many check_*() functions for raising RelaxErrors to tell the user when something is
wrong.  This will be used to simplify, make more consistent, and fix cdp.exp_type errors in the
dispersion code.
Added a number of auxiliary functions to specific_analyses.relax_disp.disp_data.

These are get_curve_type(), has_exponential_exp_type(), and has_fixed_time_exp_type() and will be
used to simplify the dispersion code.
Fixes for the specific_analyses.relax_disp.api module for the relax_disp.exp_type change.

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 loop_exp*() functions are now being used throughout the module.  The
specific_analyses.relax_disp.checks.check_*() functions are also used to simplify the code and fix
changes to cdp.exp_type.  And some auxiliary functions from specific_analyses.relax_disp.disp_data
are being used as well.