Handle multiple HALLCONF values (Resolves #19)

[SAK917]

Add functionality to handle multiple HALLCONF values in Adafruit_MLX90393.py by mimicking approach used in the Arduino library for this device (as per @caternuson):

Add comment listing valid values for HALLCONF (i.e., 0x0C or 0x00)
Add level of nesting to _LSB_LOOKUP to accommodate multiple HALLCONF values
Add lookup information for HALLCONF = 0x00 to _LSB_LOOKUP. Confirmed values against most recent datasheet (rev. 06)
In @property magnetic():

• Add check for valid HALLCONF values and raise ValueError if not 0x0C or 0x00
• Set _LSB_LOOKUP index as appropriate for HALLCONF
• Modify raw-value conversion equations to handle new _LSB_LOOKUP with additional level of nesting

NOTE: The code in property magnetic() deviates from the Arduino version in that it uses the appropriate _LSB_LOOKUP values based on the value of HALLCONF, whereas the Arduino version has the index hardcoded to [0] so it always uses the HALLCONF = 0x0C values in _LSB_LOOKUP regardless of the actual value of HALLCONF. The code to determine and use the appropriate _LSB_LOOKUP index value based on the HALLCONF value does not exist in the Arduino version.

I am unable to test this since I don't have the MLX90393 device, but am not getting any Python / Pylint errors.

[caternuson]

Thanks. This looks good. A good follow on issue/PR would be to actually add the ability for the user to set HALL_CONF via a new property. But that can be done separately. This PR gets the table updated and maintains the current behavior.