Fixes sign error in reflection_coefficient_at_theta function

This corrects a sign error in the function reflection_coefficient_at_theta. Specifically, the returned value should be  Gamma0 * exp(-2j * theta) per both transmission line theory and the function's docstring, but the actual code omits the negative sign. Note that this bug affects the results of a number of other functions, including input_impedance_at_theta (zl_2_zin).