Incorporating changes from Canary IL project.

[WolvesOfTheNight]

I made an updated version of the AD5933 library as part of a project I was working on. Here are the changes, if you are interested.

• Updated readme
• Updated comments
• Added frequency code conversion function, freqToCode
• Added 16-bit frequency sweep functions for explicit control of word size, if desired. The use of 32-bit words was consuming a lot of memory when I started working with multiple sweeps.
• Added support for setting the start and increment frequencies by code or Hz (interger only) so that users can run sub-1Hz frequency steps if desired. This is especially useful when running several sweeps to build a larger spectrum. Support for setting frequency as a float was not added because you really need to understand the limitations of the frequency code when running sub-1Hz steps.
• Adds support for averaging several tests at each data point.
• Added setOutputVoltage
• Added setSettlingCycles
• Set getTemperature to return -666 in case of an error because -1 degrees C is a valid temperature, and quite possible for some uses of the device.

I ended up doing the data analysis in my own program. I though about updating the calibration function here, but I would end up breaking backward compatibility. I might be able to come up with a new process data function if there is sufficient interest.