index: Use Math.PI over numeric constant

[kevinji]

Due to the precision of JavaScript's doubles, the two numbers are equivalent.

[kgryte]

@mc10 Thanks for the PR; however, we're not interested in reassignment. Hard coding the number ensures that the constant is the coded value. Otherwise, someone could hijack global state (e.g., create a new Math object having a constant value which is no longer correct). One purpose of this package is to explicitly avoid globals.