New function citation() to display citation information (patch #7960) * display_info_file.m: the new function is pretty much the same as news() so this new private function is their common code * citation.m: the new function * help.m: changed to use the private function with common code with citation() * CITATION: new file with citation instructions for Octave core * Makefile.am: add CITATION file into the build system and install into octetc * bootstrap.conf: make sure that the CITATION is present when installing * copy_files.m: make pkg() check and install CITATION files from packages * doc/interpreter/package.txi: document optional CITATION files for packages