Reading in header information from itp file

[csbrasnett]

I realised I buried this within #483, and might be quite ambitious, but it would be useful to read back in header information (eg. the secondary structure determined by DSSP) from an itp file generated by martinize2. It would also be useful for maintaining citation information in a file editing pipeline.

The second advance in this regard would be maintaining grouping of different intra-directive information, eg. whether bonds are backbone-backbone, or angles BB-SC-SC, etc. so that these could be edited selectively, or again, maintained when subsequently writing out.