New logging framework based on g3log

[bredelings]

Creates files like otc-solve-subproblem.log.txt, and appends to them.

By default log files are created in the current directory. The logging directory can be specified with the -l flag.

Loggers are initialized from either

• cmd_line_set_logging( ) -- if the tool is using boost::program_options
• OTCLI::parse_args( ) -- otherwise