Sampling Rate for ADC is extremely low

As currently written, it appears you can only do about 93 SPS. It looks like if there is a shorter delay introduced on analog_read, it drastically increases the sampling rate and it seems to be running stable.

`
def analog_read(self, pin):
"""Read the value of an analog pin by number"""
buf = bytearray(2)
if pin not in self.pin_mapping.analog_pins:
raise ValueError("Invalid ADC pin")

self.read(
_ADC_BASE,
_ADC_CHANNEL_OFFSET + self.pin_mapping.analog_pins.index(pin),
buf,
.0001,
)
ret = struct.unpack(">H", buf)[0]
time.sleep(0.001)
return ret
`

Start at the analog_read entry point shown in the issue and examine the delay after the ADC read. Measure the current sampling rate, then verify that a shorter delay remains stable and produces the expected improvement. The work is done when analog readings sample faster without instability.

Written by the indexing model from the issue text.