Pow() for Decimal

[`Pow()`](https://docs.microsoft.com/en-us/dotnet/api/system.math.pow) is only defined for [`Double`](https://docs.microsoft.com/en-us/dotnet/api/system.double) which is naive. The geometric mean, which is used for numerous financial calculations, and I'm sure other algorithms, make use of exponentiation of decimals. As such, it needs to exist.