This avoids unexpected failures with no or poor error messages.

Comments may consist of more than one line.  So far, only the first
line was deleted without fixing brackets thus corrupting the file.
From now on, the comments are deleted as a whole.

User expired authored May 14, 2019 and User expired committed Jun 13, 2019

Hence, bibtex file may contain the '|' character.

User expired authored Mar 23, 2018 and User expired committed Jun 13, 2019

Recent changes and improvements were motivated by extensive
use and preparing a presentation of the bibtools scripts.