digitalRead()/analogRead() interference

[avtolstoy]

Fixes #993

• Saves PinMode in PIN_MAP[pin].user_property instead of a single global variable
• Fixes ADC PinMode configuration
• Fixes DAC PinMode configuration (affected by the same issue as ADC pins described in #993 under certain conditions)

Relevant tests:

• wiring/no_fixture GPIO_AnalogReadWorksMixedWithDigitalRead
• wiring/adc_dac DAC_AnalogWriteWorksMixedWithDigitalRead

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Doneness:

• Contributor has signed CLA
• Problem and Solution clearly stated
• Code peer reviewed
• API tests compiled
• Run unit/integration/application tests on device
• Add documentation (to docs master)

analogRead():
Since 0.5.3 Note: you do not need to set the pinMode() with analogRead(). The pinMode() is automatically set to AN_INPUT any time analogRead() is called for a particular analog pin, if that pin is set to a pinMode other than AN_INPUT. If you explicitly set a pin to INPUT, INPUT_PULLUP, INPUT_PULLDOWN or OUTPUT before using analogRead(), it will switch it back to AN_INPUT before taking the reading. If you use digitalRead() afterwards, it will automatically switch the pinMode back to whatever you originally explicitly set it to.

• Add to CHANGELOG.md after merging (add links to docs and issues)

[m-mcgowan]

Tested on the Core.