Re-initialize ADC before every AnalogIn read.

[dhalbert]

microcontroller.cpu.temperature uses different ADC settings, and caused
AnalogIn to give wrong answers. AnalogIn can no longer assume it's the
only user of the ADC.

This fixes #253, at the expense of re-initing the ADC on every read, and also doing double reads to ensure accuracy of the result.

A more sophisticated implementation would remember the last ADC settings and not re-init if they were the same. If we're not going to do that, the AnalogIn code could be simplified to not save the ADC structs on the heap, but simply re-create them each time.

@tannewt -- do you have an opinion? Maybe it's not worth cleaning up because it will all change with ASF4.

[tannewt]

This works for me! Please merge it into the 2.x branch too. We'll need to redo it for ASF4 so lets not put a bunch of time into it.