Add public method to set thermocouple type

[Gadgetoid]

There's currently no public method to set the thermocouple type, this can be worked around with calls directly into i2cdevice.

For example, here's how we might set the type to 'R':

from mcp9600 import MCP9600
mcp9600 = MCP9600()
mcp9600._mcp9600.set('THERMOCOUPLE_CONFIG', type_select='R')

This should really be something like:

mcp9600.set_thermocouple_type('R')