add functions relating to ice

[dankelley]

Issue #20 has a checklist that lists all the TEOS-10 functions, and that is not suitable, given the title of that issue. Therefore, I intend to break some things out to separate issues; here, that means ice-related functions. I developed the checklist based on http://www.teos-10.org/pubs/gsw/html/gsw_contents.html

• gsw_adiabatic_lapse_rate_ice
• gsw_alpha_wrt_t_ice
• gsw_cp_ice
• gsw_chem_potential_water_ice
• gsw_enthalpy_ice
• gsw_entropy_ice
• gsw_frazil_ratios_adiabatic
• gsw_frazil_ratios_adiabatic_poly
• gsw_frazil_properties
• gsw_frazil_properties_potential
• gsw_frazil_properties_potential_poly
• gsw_gibbs_ice
• gsw_helmholtz_energy_ice (note lower-case on name, as for Gibbs)
• gsw_ice_fraction_to_freeze_seawater
• gsw_internal_energy_ice
• gsw_kappa_ice
• gsw_kappa_const_t_ice
• gsw_melting_ice_equilibrium_SA_CT_ratio
• gsw_melting_ice_equilibrium_SA_CT_ratio_poly
• gsw_melting_ice_into_seawater
• gsw_melting_ice_SA_CT_ratio
• gsw_melting_ice_SA_CT_ratio_poly
• gsw_pot_enthalpy_from_pt_ice
• gsw_pot_enthalpy_from_pt_ice_poly
• gsw_pressure_coefficient_ice
• gsw_pt0_from_t_ice
• gsw_pt_from_pot_enthalpy_ice
• gsw_pt_from_pot_enthalpy_ice_poly
• gsw_pt_from_t_ice
• gsw_rho_ice
• gsw_sound_speed_ice
• gsw_specvol_ice
• gsw_t_from_pt0_ice

[dankelley]

I am closing the issue, because I have added each of the ice-related functions listed in the first comment of this issue. (Some were not in the C library, so they are crossed out in the checklist.) Each function has documentation that has an example with the test code provided in the Matlab-oriented webpages, and since building the package requires that all tests work, I have high confidence that this work has been done well.