Add "Runner"

Simplifies the convergence workflow by allowing you to load the data,
tell the Runner which interconnects need to exist and with what
capacity. You will get the calculated merit order automatically.

Currently the Runner permits only one "true" interconnect (where import
and export depend on the costs in each country), but you may add as many
"fixed" exports (using pre-made load curves) as you wish.

See examples/all_in_one.rb for an example.
Make Archive#area return a Symbol