[j-bryan]

Closes #55. Stacked on #61.

This PR fixes an issue with interest accrual on debt. Previously, interest accrual would stop at construction_end regardless of when amortization_start (defaulting to operations_start) fell. This would mean that any gap between construction_end and operations_start was resulting in an interest-free period.

Changes:

• Adds an accrue_until optional parameter to ConstructionFinancing so a user can manually set the date at which construction debt interest stops accruing.
• AnalysisContext.amortization_start() resolves the accrual end in build_construction and the amortization start in build_debt
• An amortization_start before construction_end now raises a ValueError

For the example provided in #55, we observe the following changes in outputs:

• debt principal increases from $412.85M to $454.14M
• NPV decreases from $296.3M to $276.3M

[dylanjm]

Changes are good.