Quiet mhctools logging by default

[iskandr]

Fixes #260.

Summary:

• Stop calling logging.config.fileConfig from mhctools.logging.get_logger.
• Attach a NullHandler to the mhctools package logger so imports stay quiet by default.
• Add mhctools.set_log_level(...) as the supported top-level log threshold control.
• Bump version to 3.31.4.

Validation:

• pytest tests/test_logging.py -q
• ./lint.sh
• ./test.sh (657 passed, 51 skipped, 2 xfailed)