v3.1.0 - major rewrite to make the code more suitable for inclusion in models.

- Added ice/seaice and related functions/subroutines;

- Most functions/subroutines are now elemental (enabling calls with
  either scalars, vectors or arrays);

- constants/fixed coefficients are now defined in modules;

- implemented a prototype error handling system;
Merge branch 'master' of https://github.com/TEOS-10/GSW-FORTRAN