relax SVN archive

Copied the exponential curve fitting c code as a template for the inversion-recovery code.

The command used was:
svn cp maths_fns/exponential.c maths_fns/inversion-recovery.c

Created a new branch for the implementation of inversion-recovery R1 curve fitting.

This is the first step for the corresponding task:
https://gna.org/task/index.php?7415

This follows a thread started at:
https://mail.gna.org/public/relax-devel/2011-06/msg00138.html
Message-id: <4DF8A720.9000006@...>

Adding a new test xyz file.

This is Han's patch https://gna.org/task/download.php?file_id=13224 attached to the task
http://gna.org/task/?7404.

This is a new xyz file, which contains several different models. This file should be put in the
directory: test_suite/shared_data/structures.

Better handling of the return to the initial GUI screen.

This was broken on MS Windows.

add_start_screen() now re-performs a main window layout.

This allows for a proper layout once all analysis tabs have been deleted.

If all analyses are deleted, then the initial GUI state is set and the start screen remade.

The initial sizer with the relax icon is now properly deleted.

Fix for the R1 and R2 analysis tab initialisation.

This bug was introduced in the last commit.

The relax data store GUI analysis elements are now only initialised by the analysis classes if asked.

By passing in an index for a currently existing analysis data structure, a new structure will now
not be initialised by the analysis classes.

Fix for the from_xml() method based on the change to add() in the GUI analyses data object.