Rounding options

[zmoon]

This adds a new rounding method that aims to preserve significant figures ("sig figs") in base 10 (as opposed to decimal places), and a new function round_dataarray that can use either rounding method.

I also implemented the rounding in binary (https://github.com/fraserwg/elePyant/issues/1) following https://github.com/fraserwg/elePyant/issues/1#issuecomment-658438978, but wanted to see if/how you wanted to handle options in the compress_ functions first before I include that. My thought was that they could call round_dataarray, but arguments would need to be changed.

[zmoon]

@fraserwg what do you think about this?