[BUG] function discover_voltage_zones not allowing to apply voltage bounds to a multinetwork test case

[juanjospina]

Describe the bug
There is a line of code in the function discover_voltage_zones(..) that is not allowing the user to apply voltage bounds to a multinetwork test case.

The problem is present in line 113 of file units.jl

@assert iseng(data_model) || ismath(data_model) "unsupported data model"

In a multinetwork test case, when the function _apply_voltage_bounds!(..) -> calc_voltage_bases(..) call discover_voltage_zones(..), the data passed in does not have the data_model key so the assertion always fails. This only happens when a multinetwork test case is used.

Minimum Viable Example

case3_balanced = parse_file("./PowerModelsDistribution.jl/test/data/opendss/case3_balanced.dss")
eng_ts = make_multinetwork(case3_balanced)
apply_voltage_bounds!(eng_ts; vm_lb=0.95, vm_ub=1.2)

This simple example will fail due to the assertion. If the assertion is commented out, then everything works correctly.

Expected behavior
The function apply_voltage_bounds!(..) should be able to apply the voltage bounds to all the buses in the multinetwork test case problem.

Screenshots
N/A.

System Information (please complete the following information):

• OS: Ubuntu 20.04 LTS
• Version: 0.14.3 (main-latest)

Additional context
Possible solutions:

1. Remove the assert since may not be needed.
2. Add an extra boolean argument (kwarg) that goes all the way from apply_voltage_bounds! -> _apply_voltage_bounds! -> calc_voltage_bases -> discover_voltage_zones. If it is true, then is mn and the assertion is skipped.

If there is a preference to one of these solutions, let me know so I can try to implement it and make a PR.