Add generic frequency conversion class

[dihm]

I find I am often in need of a simple unit conversion class for frequencies that goes between standard SI prefixes. Figure this is basic enough functionality it should probably go into the mainline.

This provides the FreqConversion class that goes from a base of Hz to kHz, MHz, or GHz.

[dihm]

Going to go ahead and merge as this is a pretty innocuous change that won't effect anything existing.