Logging for debug output

Matplotlib has in the past (sporadically) used an internal verbose-output reporter. This version converts those calls to using the standard python logging library.

Support for the old rcParams verbose.level and verbose.fileo is dropped.

The command-line options --verbose-helpful and --verbose-debug are still accepted, but deprecated. They are now equivalent to setting logging.INFO and logging.DEBUG.

The logger’s root name is matplotlib and can be accessed from programs as:

import logging
mlog = logging.getLogger('matplotlib')

Instructions for basic usage are in Troubleshooting and for developers in Contributing.