Add h3ToCenterChild and getPentagonsAtRes

[zachasme]

Follows up on #263. I have a use-case where I need to find the center child of a cell at a specific resolution.

I'm simply changing the four resolution bits, but it should be worth adding as a helper function to the core library (so we can keep treating the index as an opaque datatype).

Also adds getPentagonsAtRes which provides the user with the set of pentagon cells at any given resolution,

Resolves #255 and #263.

[coveralls]

Coverage remained the same at 100.0% when pulling 4a4ad44 on zachasme:feature-add-h3tocenterchild into f6106a9 on uber:master.

[zachasme]

What are your thoughts on naming?

The atRes suffix is not used anywhere else, perhaps getPentagonalIndexes/pentgonalIndexCount would be better?

[nrabinowitz]

Thanks so much for the contribution! Looking really good, just a few comments.

[nrabinowitz]

What are your thoughts on naming?

The atRes suffix is not used anywhere else, perhaps getPentagonalIndexes/pentgonalIndexCount would be better?

We also don't use pentagonal anywhere :). I don't mind atRes but could see just getPentagons or getPentagonIndexes. No strong preference.

[isaacbrodsky]

Thanks for your PR! Looks good, I added some comments in addition to Nick's.

[zachasme]

Thank you for all the feedback! I believe I've addressed it all. Please let me know if I've missed something.

I've added static bool _isValidChildRes(int, int) which is now used in h3ToChildren, maxH3ToChildrenSize, h3ToCenterChild, uncompact and maxUncompactSize as per your suggestion.

[nrabinowitz]

This looks great - thanks for the follow-through on this PR!