fix: create copy of bus table before modifying within get_bus_demand function

[danielolsen]

Pull Request doc

Purpose

When performing the get_bus_demand function, avoid modifying the bus table of the Grid that is passed as input. This is not a problem when we call scenario.get_bus_demand() on a Scenario in Analyze state, since that method passes a fresh copy of the grid so the grid associated with the scenario doesn't change, but it's still an unintuitive behavior in case anyone uses the lower-level function.

Testing

Before:

>>> from powersimdata import Scenario
>>> from powersimdata.input.input_data import get_bus_demand
>>> scenario = Scenario(599)
>>> grid = scenario.get_grid()
>>> len(grid.bus.columns)
19
>>> get_bus_demand(scenario.info, grid)
>>> len(grid.bus.columns)
21

After:

>>> from powersimdata import Scenario
>>> from powersimdata.input.input_data import get_bus_demand
>>> scenario = Scenario(599)
>>> grid = scenario.get_grid()
>>> len(grid.bus.columns)
19
>>> get_bus_demand(scenario.info, grid)
>>> len(grid.bus.columns)
19

Time estimate

2 minutes.

[BainanXia]

Agree.