Return values of 0 and 1 are not always as expected about d3-ease

[Fil]

I think it's a bug.

For back it's just a matter of reordering the operations to avoid bad rounding.

backIn(t) is computed as t * t * ((s + 1) * t - s) where s is the "overshoot" parameter. If computed as t * t * (s * (t - 1) + t) it will return 1 for t=1, with no additional cost (same number of multiplications and additions).

For elastic, and exp the formula is not really expected to give 0 in 0 or 1 in 1 as it uses Math.pow(2,-10) instead of 0.

A solution for these would be to introduce easePow(x) = (Math.pow(2, 10 * (x-1)) - 0.0009765625) * 1.0009775171065494 (the two constants being 2^-10 and 1/(1-2^-10)) and use it internally in lieu of Math.pow(2, 10 * (x-1)). This would have an added cost of 1 multiplication and 1 addition. It also means we have to change all the tests.

Finally for sinIn there doesn't seem to be a solution other than an equality test (t === 1 ? 1 : Math.cos(…))

[Fil]

I've pushed a rather simple solution for back and for sin, and a second commit solves it for elastic and exp (but breaks all the tests).