Round non zero

[Fil]

fixes #242

[mbostock]

Should we employ the same test as d3-axis so that roundingPrecision can default to 0.5 if devicePixelRatio > 1?

https://github.com/d3/d3-axis/blob/4fc4f2bd287de20c4fc5f1da447ae1194d3acca6/src/axis.js#L39

Also, I think internally perhaps we should store the inverse rounding precision as an integer (e.g., precision = 2 if we want to round to the nearest half-pixel), so as to reduce the likelihood of floating point error.

Also, how about the shorter name scale.precision. Is that too ambiguous?

[Fil]

Agree with using devicePixelRatio and integer representation. For the name, I think precision presents the risk of being ambiguous (though we have no use for a scale's precision yet).