Add a few new functions:
Add a few new functions:

1. Calculate density from temperature, salinity and pressure using the approach adopted by GOTM
2. Calcualte salinity from conductivity, temperature and pressure using the SAL78 Fofonoff and Millard (1983) FORTRAN code
3. Calculate salinity from conductivity, temperature and pressure using the sal80.m MATLAB function as a reference.

Also add a separate tests function to clean up the structure a bit

pwcazenave/PyFVCOM