density_enthalpy_48: replace erroneous local functions
density_enthalpy_48: replace erroneous local functions

v_hat_denominator and/or v_hat_numerator were incorrect,
causing density-related variables such as rho to be incorrect.
I don't know where the error was; I simply copied in the
expressions from gsw_rho_alpha_beta, pythonized them, and
found that the previously failing Nsquared test now passes.

TEOS-10/python-gsw,rabernat/python-gsw,ocefpaf/python-gsw,castelao/python-gsw