Implement `f_3pow` function

[wtbarnes]

f_3pow is a function in the xray IDL package that provides a triple broken power law function. The complete code is here: https://soho.nascom.nasa.gov/solarsoft/packages/xray/idl/f_3pow.pro. This function is presumably useful either when forward modeling or fitting observed spectra.

Another approach would be to implement a function that models a power-law with $N$ breaks such that a separate function isn't needed for each kind of power-law.

[settwi]

Sure, we can probably modify the existing photon power law to support N breaks. We would need to think about normalization but that shouldn't be too bad.

How is it normalized in IDL? I like making the user pick a normalization energy and then setting the flux normalization there. Otherwise it can be confusing if we have a magic fixed energy like IDL does sometimes.