Implemented the following functions:  * pt_first_derivatives  * pt_second_derivatives  * CT_first_derivatives  * CT_second_derivatives  * entropy_first_derivatives  * entropy_second_derivatives  * enthalpy_first_derivatives  * enthalpy_second_derivatives
Implemented the following functions:
 * pt_first_derivatives
 * pt_second_derivatives
 * CT_first_derivatives
 * CT_second_derivatives
 * entropy_first_derivatives
 * entropy_second_derivatives
 * enthalpy_first_derivatives
 * enthalpy_second_derivatives

I'm implementing in a more similar way to the Matlab version to make future
comparisons easier to read.

pyoceans/python-seawater,ocefpaf/python-seawater,pyoceans/python-seawater,ocefpaf/python-seawater