LArPix-v2b upgrade

[peter-madigan]

The LArPix-v2b chip is arriving soon and so larpix-control needs to be ready to communicate with the new chips. In particular,

1. There will likely need to be a new configuration class under larpix/configuration/, it should be able to inherit most functionality from the Configuration_v2 class.
2. There will need to be a new default configuration under larpix/configs/chip.
3. The Chip class under larpix/chip.py will need to know about the new configuration class.
4. Controller.load will need to have a switch for the new asic version.
5. There may need to be a new Packet class under larpix/packet if the uart protocol has changed

[peter-madigan]

Also don't forget to add a test suite in test/ to make sure it works as you expect!