Add log function to syncopy root namespace

[dfsp-spirit]

Changes Summary

• Adds a syncopy.log() function that can be used to log without first importing stuff from submodules, and supports both the sequential and the parallel loggers:

import syncopy as spy
spy.log("We are about to get real.", level="IMPORTANT")
spy.log("oh no, an error in parallel code!", level="ERROR", par=True)

Reviewer Checklist

• Are testing routines present?
• Do objects in the global package namespace perform proper parsing of their input?
• Are all docstrings complete and accurate?
• Is the CHANGELOG.md up to date?

[tensionhead]

Great, with this spy.log() can be conveniently used to drop persistent messages!