check current limits

[frederikgeth]

• whether they are defined in the same way for all formulations
• add new problem type for current-limited equivalent

[frederikgeth]

Consider making the approach consistent with lanl-ansi/PowerModels.jl#354

[frederikgeth]

We should address this asap:

• opf problem type purely apparent power limits dominating
• opf problem type with current limits dominating