Fix number interpolator ending value for very large a

[yamass]

Current situation:

When a number interpolator is applied to a very large floating point number starting value, the ending is 0.

var i = interpolate.interpolateNumber(2e+42, 335);
i(1.0) === 0 // !

This is due to floating point number arithmetic errors:

var a = 2e+42;
var b = 335
var delta = b - a;
delta === -2e+42 // !

Fix

There is IMO no perfect fix for the calculation for 0<t<1,
but for t===1, the ending value should be returned (without any calculation).

[mbostock]

We should fix this, but we should fix not just interpolateNumber but all the other interpolators at the same time. I’ve filed #73 to track with my proposed formula.

[mbostock]

Superseded by #76.