Add keyword arguments to spectrogram

[v923z]

Details and rationale are outlined in https://github.com/v923z/micropython-ulab/blob/spectrum/docs/ulab-utils.ipynb, but the gist is that spectrogram now allows for the re-use of allocated memory (through the out, and scratchpad keyword arguments), as well as direct calculation of the logarithm (through the log keyword argument).

from ulab import numpy as np
from ulab import utils as utils

n = 1024
t = np.linspace(0, 2 * np.pi, num=1024)

scratchpad = np.zeros(2 * n) # re-usable RAM for the calculation of the FFT

for _ in range(10):
    signal = np.sin(t)  # this simulates a measurement of some signal
    utils.spectrogram(signal, out=signal, scratchpad=scratchpad, log=True)

@jepler, @dhalbert this might be relevant for circuitpython users.