Add merge_offset_ranges utility

[rjzamora]

This implements a simple merge_offset_ranges utility to support the max_gap option for the cat_ranges method added in #744. Note that this utility also recognizes a max_block= argument to help preserve parallelism for large reads. I intend to use this utility (via cat_ranges) to help improve remote-file-system performance in Dask.

[martindurant]

Great to see this!

I have the one API question that I think needs to be answered.
Note that even without solving that question, the code would already be useful, but require the called to themselves call marge_offset_ranged, and then cat_ranges with the result (i.e., not actually use the max_gap arg of the latter).

[martindurant]

Thank you for the changes, @rjzamora !

Can I please request that you remove the copyright and licensing blocks, since this whole project is already licensed as BSD-3.

That aside, this is good to go. I was about to say you should add the to the API docs, but maybe that can wait until we see any usage aside from our own.

[rjzamora]

Can I please request that you remove the copyright and licensing blocks, since this whole project is already licensed as BSD-3.

Sounds good. Thanks for the review @martindurant !