Implement `microphone.set_sensitivity(gain)`

[microbit-carlos]

As discussed in:

• WIP: Audio recording and playback #163 (comment)

The following values can be provided in the microphone module:

• SENSITIVITY_LOW = 0.079
• SENSITIVITY_MEDIUM = 0.2 (the default)
• SENSITIVITY_HIGH = 1.0

e.g. microphone.set_sensitivity(microphone.SENSITIVITY_HIGH)

[dpgeorge]

Now implemented in the branch/PR.