Allow custom loggers

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)?

Start by reading the linked SUNErrHandlerFn documentation and locating the current logfile configuration and logging implementation. Determine where the refactoring removed callback-based logging and what existing tests cover; done means the issue's requested custom logging behavior is defined and verified without losing filename-based logging.

Written by the indexing model from the issue text.