relax SVN archive

This is a minor feature and bugfix release. The new user functions system.cd and system.pwd have
been added to allow the working directory to be changed and displayed. The time and sys_info user
functions have been renamed to system.time and system.sys_info. The structure.delete_ss user
function has been created to remove the helix and sheet information from the internal structural
object. For bugs, the 'R2eff' dispersion model can now handle missing peaks in subsets of spectra,
and the structure.read_pdb can now handle multiple structures and multiple models with the merge
flag set.