Add Raspberry Pi sensor observables

[whymarrh]

Closes #53

This PR implements observables for CPU voltage, frequency, and temperature (via Pi4J's SystemInfo methods). The usage is simple enough; a 1 minute poll interval:

final CpuInformation cpu = new CpuInformation(1, TimeUnit.MINUTES);
cpu.observe().subscribe();

[cal-pratt]

ship it!

[cal-pratt]

Has this been tested? Can it be merged into master?

[whymarrh]

This has been tested and it works, but I have a few changes to make before merging this.

[whymarrh]

Should the API expose the three methods that are there now, or should there be just one Observable<CpuInformationValue> that has props for voltage, frequency, and temperature?

[cal-pratt]

The latter. It would be cool to have a CpuInformation value pair with a String var for hostname ("rasprime", "picam1", etc). Then we could pretty easily make a view on the GUI to monitor all the different pi's.

[whymarrh]

This needs to sanity-checked on a Pi, but is ready for review /cc @EasternEdgeRobotics/developers

[cal-pratt]

If we want to observe multiple raspi we will need a name field

[whymarrh]

Where the images don't have their hostnames set to anything meaningful, would it be better to attach a name externally instead? That is, in each entry point, when wiring up this information, we can map machines to the streams (somehow).

(Which might be doubly useful if we need to map machines to other information.)

[cal-pratt]

We could make our own composite class to wrap names with an object? Essentially a clone of the timestamped class you mentioned the other day.

[whymarrh]

That would work, yeah. How does that sound?

[cal-pratt]

👍

[cal-pratt]

Ship it