hawc2/htc: Order of commands within new_htc_structure

In general, the order of the contents of an HTC file is preserved when read by HTCFile() and then saved/printed. However, within the new_htc_structure block, the main_body sections always come first. My preference would be to also allow that the various structural analysis commands (Seen here) could precede the main_body blocks, if that is the order in the original contents.

It appears that the re-ordering occurs here.

A suggestion could be to always put constraints last, rather than always put main bodies first.