Actually calculate dc voltage bounds

[adammate]

core/data.jl/...: properly/actually calculate dc voltage bounds (currently they return a large number; Inf is not possible due to recent JuMP changes) => function calc_min_dc_voltage; function calc_max_dc_voltage; function calc_max_dc_voltage_difference

[jtabarez]

A starting point could be sum of absolute values of all coupled transmission line voltages in the system