v0.16.0 add offgrid capability, many name changes

Allows users to model "off-grid" systems as a year-long outage:

• add flag to "turn on" off-grid modeling Settings.off_grid_flag
• when off_grid_flag is "true", adjust default values in core/ electric_storage, electric_load, financial, generator, pv
• add operating reserve requirement inputs, outputs, and constraints based on load and PV generation
• add minimum load met percent input and constraint
• add generator replacement year and cost (for off-grid and on-grid)
• add off-grid additional annual costs (tax deductible) and upfront capital costs (depreciable via straight line depreciation)

Name changes:

• consistently append _before_tax and _after_tax to results names
• change all instances of timestep to time_step and timesteps to time_steps

Other changes:

• report previously missing lcc breakdown components, all reported in results/financial.jl
• change variable types from Float to Real to allow users to enter Ints (where applicable)
• year_one_coincident_peak_cost_after_tax is now correctly multiplied by (1 - p.s.financial.offtaker_tax_pct)