Deduplicate calls to `calculate_pre_revenue_costs_and_cashflow`

[softwareengineerprogrammer]

Per #444 (comment):

In EconomicsSam.py, calculate_pre_revenue_costs_and_cashflow is called multiple times both pre- and post-single owner model execution. This implementation pattern was used because it is a convenient way to reference the pre-revenue calculation results in multiple places without a potentially unwieldy amount of passing it around as a result object. However, it should ideally really only be called once, pre-execution and not afterwards (such as here) both for code clarity and to reduce the potential for errors if the assumption that results remain the same pre- and post-execution is ever invalidated.