relax SVN archive

Created the new '-r' or '--traceback' relax command line option.

When supplied, then stack tracebacks will be shown for all RelaxWarnings and RelaxErrors. This
allows for finer debugging control.

Created the lib.warnings.TRACEBACK variable.

When True, this will cause stack-traceback to be printed out with the warning. This is to decouple
the traceback printout from the warning to error escalation.

Converted the status.escalate variable into module variables for lib.errors and lib.warnings.

This variable is set by the command line flag '-e' or '--escalate'. By converting it into a module
variable, the lib.errors and lib.warnings warnings are now independent of relax.

Implemented linear constraints for the frame order analysis.

This uses the log-barrier constraint algorithm in the minfx library https://gna.org/projects/minfx/
to provide constraints without requiring gradients.

Docstring fix in the relaxation curve-fitting linear_constraints() function.

The pivot point x, y and z coordinates are now registered as parameters of the frame order analysis.

These are stored as the parameters pivot_x, pivot_y and pivot_z.

Docstring update for the relaxation dispersion linear_constraints() function.

Fixes for the frame_order.pivot user function - the model parameters were not being updated.

The update_model() function is now called to make sure that the pivot point is either added or
removed from the list of model parameters.

Changed the title for the value user function class.

Changed the title in the value.write user function documentation.