specvol() 75-term (#31)

specvol() based on 75-term equation

doctest values obtained from GSW-Matlab 3.0.5

Temporarily disabling test_profile on specvol. This is not the best way to do it, but test_profile was meant for 48-term and looks like it wasn't validated for Matlab's specvol anyways. Therefore, approval of test_profile() doesn't guarantee it is correct anyways.

Once there are more converted functions, it would be a good idea to update and reactivate test_profile for specvol().

Author: castelao

gsw/gibbs/__init__.py

@@ -13,6 +13,7 @@
 from .water_column_48 import *
 from .practical_salinity import *
 from .density_enthalpy_48 import *
+from .density_enthalpy_75 import *
 from .neutral_nonlinear_48 import *
 from .thermodynamics_from_t import *
 from .density_enthalpy_48_ct import *

gsw/gibbs/constants.py

@@ -97,7 +97,12 @@
 
 sfac = 0.0248826675584615
 """
-sfac = 1 / (40. * (SSO / 35.))
+sfac = 1 / (40 * uPS) = 1 / (40. * (SSO / 35.))
+"""
+
+soffset = 5.971840214030754e-1
+"""
+soffset = deltaS * sfac.
 """
 
 R = 8.314472