Add helper functions to enable and disable BURBLE to print diagnostics.

[eriknw]

Adds enable_burble and disable_burble. I want approval before merging this. Heh, I couldn't help myself: burbling is too fun a name to pass up!

We could also have this as e.g.

>>> from suitesparse_graphblas import burble
>>> burble.enable()
>>> # Do stuff
>>> burble.disable()
>>> with burble:
...     # Do some more stuff

Is it worth it as a context manager? Should it restore to the previous value or GxB_BURBLE instead of disabling it?

[eriknw]

@jim22k voted to have burble as an object with burble.enable(), burble.disable(), and the ability to be used as a context manager, so I updated the PR to do this.