Add synthio.MidiTrack.tempo property

[relic-se]

This update allows the tempo of a synthio.MidiTrack object to be manipulated after it has been constructed.

In my use case, I wanted to gradually increase the speed of a song within a game as it progresses.

Here is a demonstration of this property written for the Fruit Jam:

import audiobusio
import audiomixer
import board
import synthio
import time

import adafruit_tlv320

# configure dac
i2c = board.I2C()
dac = adafruit_tlv320.TLV320DAC3100(i2c)
dac.configure_clocks(sample_rate=44100, bit_depth=16)

# use headphones
dac.headphone_output = True
dac.headphone_volume = -15  # dB

# setup audio output
audio = audiobusio.I2SOut(board.I2S_BCLK, board.I2S_WS, board.I2S_DIN)

# load melody
melody = synthio.MidiTrack(b"\0\x90H\0*\x80H\0\6\x90J\0*\x80J\0\6\x90L\0*\x80L\0\6\x90J\0" +
                           b"*\x80J\0\6\x90H\0*\x80H\0\6\x90J\0*\x80J\0\6\x90L\0T\x80L\0" +
                           b"\x0c\x90H\0T\x80H\0\x0c\x90H\0T\x80H\0",
                           tempo=640, sample_rate=dac.sample_rate)
audio.play(melody, loop=True)

# increase tempo by 2% every second
while True:
    time.sleep(1)
    melody.tempo = int(melody.tempo * 1.02)

[tannewt]

Looks great! Thanks!