set speaker and headphone volume levels during init.

[FoamyGuy]

I noticed by default that speaker_volume after DAC init is 63.1579 which is outside the range specified by the speaker_volume property.

I don't think this setting is actually taking effect because the sound output from the speaker is still fairly quiet.

However I think it's best to explicitly set both of these to a value we know is safe for the speakers that are stocked in the shop. Both to avoid any potential damage to hardware, and to make it less confusing if anyone happens to check the value of and assumes that the value it returns will be valid for passing back to it to be set.

[dhalbert]

I think there is something wrong with the speaker volume getter or setter:

>>> t = adafruit_tlv320.TLV320DAC3100(board.I2C())
>>> t.speaker_volume
63.1579
>>> t.speaker_volume = -15
>>> t.speaker_volume
-15.7895
>>> t.headphone_volume
-63.5
>>> t.headphone_volume = -15
>>> t.headphone_volume
-15.0                     # looks ok
>>> t.speaker_volume = 0
>>> t.speaker_volume
0.0
>>> t.speaker_volume = 63.1579
>>> t.speaker_volume
0.0                         # hunh?
>>> t.speaker_volume = 50
>>> t.speaker_volume
0.0
>>> t.speaker_volume = 20
>>> t.speaker_volume
0.0