Implement support for early retirement

[josiahjohnston]

• Implements support for early retirement in generators.core.build, which had been on our wish list for several years. An optional extra column in gen_build_predetermined.tab can selectively enable early retirement for pre-determined builds, and gen_can_retire_early defaults to False.
• By default, the amount of capacity to retire is a linear decision, but can be discretized as all-or-nothing via the --enforce-binary-retirement command-line option.
• Early retirement eliminates fixed O&M costs, but does not impact capital cost expenditures.
• Once capacity is retired, it cannot be brought back online in future periods (no "mothballing" in this implementation)

In some modeling problems, early retirement is the only way to allow old baseload coal or nuclear plants to shutdown completely, which can be necessary to meet environment regulations on emissions or cooling water.