Add public method to set thermocouple type

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')
```