Piped ability to set the solver, added networks to the carbon tests

            setattr(self, name, variable)

# class NetworkModelWithTotalCarbon(NetworkModel):

#     """

#     A subclass that allows connections between hubs.

#     """

#     MAX_CARBON = 0

#     def __init__(self, *, excel=None, request=None, name=None, n=0, network=None, network_request=None, hub_id=None):

#         super().__init__(excel=excel, request=request, name=name, network=network, network_request=network_request, hub_id=hub_id)

#         if n is not None:

#             self.MAX_CARBON = n

#     @constraint()

#     def max_carbon_level(self):

#         """

#         A constraint for epsilon constraint method.

#         """

#         return self.total_carbon <= float(self.MAX_CARBON)

# NETWORK Link specific constraints:

@constraint()

        yield power[t] == 1/reactance * (angle_from[t] - angle_to[t])

def multiple_hubs(minimize_carbon=False, output_filename=None, input_files=None, network_excel=None, network_request=None, carbon_value=0, n=0):

def multiple_hubs(minimize_carbon=False, output_filename=None, input_files=None, network_excel=None, network_request=None, max_carbon=None, n=0, solver='glpk'):

    """

    Core function for solving of multiple PyEHub models.

    """

    if carbon_value is None:

        carbon_value = 0

    #TODO: Put the network request logic here

    # The default excel file for network, in the network directory

    if network_excel is None:

        network_excel_file = 'Input_files/network_input/network.xlsx'

            file_name = f'{input_files}{i+1}.xlsx'

        excel_file = file_name

        if minimize_carbon:

            hub = NetworkModelWithTotalCarbon(excel=excel_file, name=f'hub{i+1}', n=carbon_value,

                                              network=network_excel_file, hub_id=i)

        else:

            hub = NetworkModel(excel=excel_file, name=f'hub{i+1}',

        hub = NetworkModel(excel=excel_file, name=f'hub{i+1}', max_carbon=max_carbon,

                                            network=network_excel_file, hub_id=i)

        hubs.append(hub)

        objective += hub.objective

    # Now solve this model.

    status = pylp.solve(objective=objective, constraints=constraints, minimize=True)

    status = pylp.solve(objective=objective, constraints=constraints, minimize=True, solver=solver)

    #: Define the different sheets that we output

    sheets = [