[Refactor] AnalogIO->AnalogIn

[brentru]

This pull request:

• Replaces analogio.proto with analogin.proto to properly define its operation in WipperSnapper.
• New Feature - Pins belonging to an ADC (for example) may optionally have their own reference voltage (ref_voltage) rather than relying on the platform default.
• New Feature - Pins belonging to an ADC may optionally set a gain to boost signals.

Resolves #191

[brentru]

On WipperSnapper Arduino, this refactor is part of the existing expander refactor (PR #913)

[tyeth]

As discussed today, it might make more sense to shift Gain to be a components custom properties rather than have to define each possibility in this enum ahead of time.
The ref voltage still makes sense here.

From Loren's summary of our talk:

basically we may want to expose a kind of language of custom properties that we can:

• implement however you want in the per-component firmware code
• add an arbitrary list of options/inputs to the json for that specific component
• web form draws it some way or other, stores selection in db
• protobufs have an optional sidecar for carrying this data
• firmware knows it will get one of the exact options that were put in the json

[brentru]

Related: #193

[brentru]

@tyeth Your comment above has been addressed by #197 which is now merged into this branch