split out external links

[Christian-B]

This only affects the python representation of external links

FPGALinks Done
spinnaker_links Todo before merge

Advantages.

1. Removes Spin1 only stuff out of the shared Machine Object
2. TODO - Removes Spin1 only stuff from the Spin2 Version objects
3. Avoids calculating link for the 99.99% of runs that do not use them
4. Example of how Spin2 links to the external world could be done

ToBe dettermined.

• Should there be a shared Super Class of links.
  • There was not in Spin1
  • Depends on what Spin2 external links need

Disadvanagtes

• Small code change to users stuff that use these links
• Ugly delayed imports