Evaluate moving `PadGrid.cuh` functionality to NanoVDB

[swahtz]

Does this functionality fit more naturally in NanoVDB as opposed to fVDB?

Originally posted by @matthewdcong in #710 (comment)

I think this would be useful for code deduplication and sharing functionality. Have to evaluate with the wider group if this style of padding is generically useful in Nano

[swahtz]

Discussed with @sifakis during the review of #712 and he agrees on this point
#712 (review)