Add context manager support for config variables

[Sean M. Law]

One minor annoyance when changing variables like config.STUMPY_EXCL_ZONE is needing to remember to revert this variable back (where appropriate). Perhaps, this can be alleviated with the use of a context manager that can help keep track of this and to perform the reversion for you once you leave the context manager.

We should do this for some of our tutorials (assuming it works)