Add support for MCP3421

[dotpointer]

MCP3421 is 18-bit but only 1 channel.

I succeeded to use your library with a MCP3421 module by commenting out all channels but 1 in the example, here are the relevant lines that I needed to change:

addr68_ch0 = MCP342x(bus, 0x68, channel=0, resolution=18)
...
adcs = [addr68_ch0]

With these changes I could read data from the module. So you can add support for the board, it works. 😃

[ouccoffee]

I think you should talk about it in python-MCP342x， but not here.