Callback function for reporting progress

[ChristopherHogan]

One thing we might think about is having a callback function for progress instead of a hard-coded printf. That is, you would call something like progress_update(label_string, fraction) and it would default to a printf.

This would allow us to potentially call something like the progressbar2 package in Python (or tqdm or similar).

Originally posted by @stevengj in #849 (comment)

[ChristopherHogan]

Helpful links:

• Pymeep installation instructions

• C++ printf callback, call from here when set.

• Definition of the current callback, set here here, which just writes to Python's stdout.

[ChristopherHogan]

https://github.com/kuk/log-progress
https://jgardiner.co.uk/blog/jupyter_progress_bar