Rework DLPoly parser to support more diverse inputs

[oerc0122]

Description of work Rewrite DLPoly FIELD and HISTORY parser to improve maintainability in line with LAMMPS parser.

Fixes

• Can load awkward file (Fixes #794)
• Can parse "bonds" from DLP files (but not ANGLES+). Should probably make these RIGIDs rather than BONDS?
• Add more powerful strip_comments function.

To test New test (dlp-awkward) containing data files from #794