Updating PEGTL

Hi all! I’ve updated the CMB superbuild’s version of PEGTL from 2.7.1 to 2.8.3. This also required a small change to SMTK. This was merged so that the CI images generated tonight have a chance at including

Be aware that in this new version of PEGTL the input classes have renamed the line_as_string() method to line_at(). If you have code that depends on PEGTL, see SMTK MR 2897 for an example of how to make your code work with either version.