Change seawater with gsw

[b-barton]

The seawater library gives a warning that is should be replaced with gsw.
"/gpfs/vkamino_new/work/benbar/PyBDY/pyBDY/src/pybdy/utils/gcoms_break_depth.py:14: UserWarning: The seawater library is deprecated! Please use gsw instead.
import seawater"

Looks like seawater.dist() is gsw.distance() but doesn't have phase angle (which we don't use anyway). So should be an easy swap. Update environment.yml