Fix calculus.pointDiff test
Fix calculus.pointDiff test

Using two points at a distance 2*Number.MIN_VALUE won't work.
This commit uses 1e-15 instead of Number.MIN_VALUE, since that's the highest precision we can get
without using an external library in javascript (possible option is to use bignum lib)

xuewei619/numbers.js,JunaidPaul/numbers.js,numbers/numbers.js,SaintDako/numbers.js