Allow custom loggers

[aragilar]

It looks like as part of the refactoring around logging/error handling, the option of being able to specify a function to control where logs go has disappeared. Currently, you can just set a filename to use, whereas previously you could tie it into other logging systems (e.g. Python's logging), or just drop the logs as needed (which you can sort-of do now by configuring the logfile as /dev/null). Would it be possible to have a logging version of https://sundials.readthedocs.io/en/latest/sundials/Errors_link.html#c.SUNErrHandlerFn (which appears not to be called for lower priority logs)?