Calculate transformer costs based on per-MW values

[danielolsen]

Purpose

Address to-do-list item from #297: calculate transformer upgrade costs on a per-MW basis, rather than per-transformer.

What is the code doing

Real functionality updates are in investment_costs.py:

• we load a CSV for transformer costs, which is upper-triangular only, and mirror it across the diagonal
• we find the closest indices (kV rating of 'from' bus, kV rating of 'to' bus) of the transformer cost table, and multiply this per-MW cost by the capacity upgrade.
• the private function select_kV is removed (functionality rolled into the main body), since we don't use it for transformers anymore, just lines.

Everything else is data, data locations, and test updates.

Time to review

15 minutes.

[danielolsen]

The transformer cost calculation method has been updated to use the regional multipliers for AC lines (using the highest voltage from either the from_bus or to_bus side of the transformer). The code is ugly, but the logic works for both the unit tests as well as large upgrade scenarios.

[BainanXia]

Thanks for updating the test.