Rename 'gmx tpbconv' to 'gmx convert-tpr'

This should help people find the tool they are looking for, rather
than one that is named for a file format that hasn't been written in
over 5 years. Updated comments and documentation accordingly.

Uncrustified, added copyright headers.

Change-Id: I5c29c30af54b90081cfb44f5b24764c8280c49fa