Logging/diagnostics support

[blackwer]

Class has literally zero reporting facilities. Use spdlog or other logging to report status if needed.

[blackwer]

See #32 for notes about useful diagnostic info.

a diagnostic output flag option that sends summary to stdout about: how many boxes used, max tree depth, total # func queries used, time taken, threads used, etc.
another diagnostic flag that dumps (to file or stdout) a list of the coords and vals of the func queries. Then someone could read this file and plot it to show the box structure and where baobzi struggled.
robustness (gracefully reporting that max-depth was reached, and not to trust the func eval in the following box regions...) when function appears to have a singularity.
you might want to look at SGJ's hcubature on github - the interface not the inner workings, and the interface for adaptive integration in matlab (quad2d, integral2) eg some of its reporting and failure handling.