[Please speak up if you have any objections, otherwise I'll go ahead with this after lunch.]

Each log message has a particular severity level e.g. DEBUG. Loggers can be configured to only process messages at and above a level e.g. INFO by default, currently with the "severity threshold" of the logger.

While writing logging demos I've formed the opinion that using "severity threshold" to specify this in function names is long winded.

I propose to rename functions such as rcutils_logging_set_logger_severity_threshold to rcutils_logging_set_logger_level, and update wrapping in rclpy to match.

This will match terminology in ros1, python logging module and log4cxx.