Bidirectional tx builds

[josiahjohnston]

This is a working implementation of bi-directional transmission builds, which is a non-controversial and tractable subset of issue #122

As I was working on this, I came up with other items for a wish list, but didn't implement them because they weren't necessary for a minimal viable prototype and I wasn't sure if we would have consensus on their desirability.

• Consolidate the cost component inputs into a single capital cost per transmission line direction ($/MW-capacity). Have users estimate cost of upgrading corridors using whatever method makes the most sense for them. Provide suggestions based on distance and a default value of $1000/MW-km.
• Rename parameters to use the prefix of tx instead of trans for brevity, and normalize existing_trans_cap to tx_existing_cap
• Investigate shifting TRANSMISSION_LINES to a 2-dimensional set of zone_from, zone_to. If Pyomo gives us the choice of using tx or (z_from,z_to) when iterating & indexing, that would be a win-win for readability and simplicity.

[josiahjohnston]

I included another commit to support optional pre-determined transmission builds. It's something I need for a project and seemed uncontroversial, so I'm rolling it into this pull request.