Summary

This PR adds zsensor.Sensor.attr_set() and zsensor.Sensor.attr_get() methods that are
necessary to set various sensor attributes if they haven't been set statically in the device tree.

I needed this because the LSM6DS3TR-C sensor in my XIAO BLE NRF52840 SENSE board wouldn't
work with zsensor because it didn't have any default configuration for sampling frequency.

I added the various SENSOR_ATTR_* constants from zephyr/incude/zephyr/drivers/sensor.h as
ATTR_* constants in zsensor.

I also added the below test script to the README.md.

Testing

Here is a test script that I ran on my board:

    from zsensor import *
    sensor = Sensor('lsm6ds3tr_c')  # name from Devicetree
    # Set full-scale to 2g (19.613300 m/sec^2)
    # units are micro-m/s^2
    sensor.attr_set(ACCEL_XYZ, ATTR_FULL_SCALE, 19, 613300)
    # Set sampling frequency to 104 Hz
    sensor.attr_set(ACCEL_XYZ, ATTR_SAMPLING_FREQUENCY, 104)
    sensor.measure()
    sensor.get_float(ACCEL_X) # -0.508 (m/s^2)
    sensor.get_float(ACCEL_Y) # -3.62 (m/s^2)
    sensor.get_float(ACCEL_Z) # 9.504889 (m/s^2)

Trade-offs and Alternatives

This adds a small amount of code size to the zsensor module; the whole module may be eliminated
by not #defining MICROPY_PY_ZSENSOR.