Add resolution, filter, and oversampling

[caternuson]

For #20 and should fix #21.

Tested with Itsy M0:

Adafruit CircuitPython 5.3.1 on 2020-07-13; Adafruit ItsyBitsy M0 Express with samd21g18
>>> import board
>>> import adafruit_mlx90393
>>> mlx = adafruit_mlx90393.MLX90393(board.I2C())
>>> mlx.magnetic
(-29.1, -27.0, -51.062)
>>> mlx.filter
7
>>> mlx.oversampling
3
>>> mlx.filter = adafruit_mlx90393.FILTER_3
>>> mlx.filter
3
>>> mlx.magnetic
(-28.95, -26.25, -51.062)
>>> mlx.oversampling = adafruit_mlx90393.OSR_1
>>> mlx.oversampling
1
>>> mlx.magnetic
(-29.55, -25.65, -52.756)
>>> mlx.resolution_x
0
>>> mlx.resolution_x = adafruit_mlx90393.RESOLUTION_19
>>> mlx.magnetic
(-27.646, -27.45, -49.61)
>>> mlx.resolution_y = adafruit_mlx90393.RESOLUTION_19
>>> mlx.magnetic
(-32.454, -25.242, -50.578)
>>> mlx.resolution_z = adafruit_mlx90393.RESOLUTION_19
>>> mlx.magnetic
(-31.252, -28.848, -52.272)
>>> mlx.resolution_x, mlx.resolution_y, mlx.resolution_z
(3, 3, 3)
>>> 

[ladyada]

looks good and i trust ya that it works. plz black and then you can merge!
hopefully fewer people will use this chip once the 395 breakout is available, as the 395 has more-sane-i2c

[ladyada]

lol