Costs (closes #6 #7 #12)

[grgmiller]

Fixes #6, #7, #12

Updates to how costs are calculated in the model.

• Updates the uniform_series_to_present_value function in financials.py to use the formula for the present value of an annuity due (Fix financial discounting formula #12)
• In generators.extensions.storage, removes the PPA cost discount for hybrid energy storage dispatch (Fix cost calculation for hybrid energy #6)
• Updates the summary_report.ipynb to show a plot of nodal costs, and display hybrid storage charging/discharging as part of storage, rather than the paired resource
• In summary_report, fixes the generator cost per MWh table. For hybrid resources, the congestion cost of the ES component nets out the congestion cost of the RE component, so there is no energy arbitrage cost associated with hybrids anymore. Energy arbitrage could be shown if the calculation is reconfigured, but it seems that this is not important.