PWM First draft

[ghost]

Description

This PR adds support for PWM output on Quail, Netduino3, STM32F429i-Disco and STM32F411 boards.

It is still missing :

• sanity checks
• support for the other reference boards
• support of Polarity.Low

How Has This Been Tested?

Tested on Quail and F429i-Disco. Netduino3 has same MCU as Quail so it should work as well.
Not tested on IngenuityMicro Electron board (F411)

Screenshots

20 KHz PWM output on Quail, yellow trace on socket #2 polarity high 20% duty-cycle, blue trace on socket #3 polarity low 80% duty-cycle.

PwmController PWM = PwmController.FromId("TIM4");
PWM.SetDesiredFrequency(20000);

pwm2 = PWM.OpenPin(3*16+15);        // Quail PD15-PWM2
pwm2.SetActiveDutyCyclePercentage(0.2);
pwm2.Start();

pwm3 = PWM.OpenPin(3 * 16 + 13);    // Quail PD13/PWM3
pwm3.SetActiveDutyCyclePercentage(0.8);
pwm3.Start();

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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Thank you for your contribution!

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[CLAassistant]

All committers have signed the CLA.