linkers not recognized in function if reduce_function_calls_flag=0

[erik-roberts]

this issue arose when using the dynasim GUI, which has reduce_function_calls_flag=0, vs the default of dsSimulate where reduce_function_calls_flag=1. the issue is that the linker variable name is not a function argument. however, it doesn't work always well to have the linker be called in the ODE, monitor, and initial conditions, and leave a non-linker argument for the function.

an example is a fn:
func(x) = x + @linker