Support for Duals of All Constraints

[ccoffrin]

Consider adding support for reporting duals of all constraints as per the discussion in #409.

[jbarberia]

It seems reasonable to replace constraint with lambda or lam in the naming convention for duals. For example:

• constraint_power_balance duals becomes lambda_power_balance_real and lambda_power_balance_imag.
• constraint_ohms_y_to duals becomes lambda_ohms_y_to_real and lambda_ohms_y_to_imag.
• constraint_power_generation_real becomes lambda_power_generation_real

Would adding an snippet like this to each constraint function in constraint_template.jl and in form/*.jl files be sufficient to implement this?

if _IM.report_duals(pm)
    sol(pm, n, :bus, i)[:lambda_power_balance_real] = cstr_p
    sol(pm, n, :bus, i)[:lambda_power_balance_imag] = cstr_q
end