Factor out estimate_external_cost_by_technology and estimate_total_cost

[chrisnatali]

And remove dependency on networker, sequencer.

The last 4 functions from MAIN_FUNCTIONS shouldn't be needed once this is done:

infrastructure-planning/estimate_electricity_cost_by_technology_from_population.py

Line 175 in 8598787

assemble_total_mv_line_network,