TransferWithCosts: Shape of the variable of the combination function is always single element array

[jvesely]

$ python3 -c 'import psyneulink as pnl; f = pnl.TransferWithCosts(enabled_cost_functions=pnl.CostFunctions.ALL); f(10); print(f.combine_costs_fct.defaults.variable)'
[0]

the above example combination function takes 3 inputs, but it's not reflected in the shape of the default variable of the combination function.