Implement microphone.sound_level_db() method.

[dpgeorge]

This returns a floating-point number in dB.

Addresses issue #170.

[dpgeorge]

Note that microphone.sound_level() returns an integer, whereas I think a float is more appropriate for the return value of microphone.sound_level_db().

Also, do we maybe need to add microphone.set_threshold_db()?

[microbit-carlos]

Thanks Damien!

Note that microphone.sound_level() returns an integer, whereas I think a float is more appropriate for the return value of microphone.sound_level_db().

Sounds good 👍

Also, do we maybe need to add microphone.set_threshold_db()?

Probably not at this stage, so far when people have asked for this feature they haven't mentioned the high/low thresholds.
We can wait until users ask set_threshold_db() and implement it at that point.

[microbit-carlos]

Tested and works great, thanks Damien!