review cfg down_cores, down_chips, down_links

[Christian-B]

Questions.

1. Do we still want to support this functionality?
2. In down_cores do we continue to support both physical, virtual downs, both or neither

If we want to continue to support physical down core we will have to.
a. Pass this down into Chip and not guess the virtual one
b. Support Spin2 2D processor addresses

If support virtual down cores why?

[rowleya]

Doing this can be useful when you can't update the blacklist on a real machine. It can also be useful when you have a real machine but can't access it right now, but still want to work on other things e.g. placement or routing, and then it is useful to be able to copy a real machine (hence also support in virtual machine).