Add gen HHV input, few bug fixes

What's Changed

• Fix bug causing ERP API to not work with CHP/prime gen by @hdunham in #208
• Input descriptions and small bug fixes by @adfarth in #206
• Make generator higher heating value an input by @hdunham in #209
• Add gen HHV, ERP bug fix, add to input descriptions, fix batt_roundtrip_efficiency calculation, push! not append() array in cost_curve.jl by @hdunham in #211

Full Changelog: v0.29.0...v0.30.0