Step counter app

[MatthewWilkes]

The accelerometer chip has a built-in step counter.

• Expose step counter API to python
• Step counter app
• Ability to publish step data somehow, somewhere

[tomsci]

Have added accelerometer module and basic UI app, but the step counter API doesn't work. We've tried a bunch of different configs (and the code currently on main is one such config that doesn't work) but so far it's not clear how to configure the QMA7981 step counter support.