*Refactor MOM_EOS_UNESCO.F90

  Refactored the expressions in MOM_EOS_UNESCO.F90, adding parentheses to
specify the order of arithmetic, starting with the highest-order terms first for
less sensitivity to round-off.  Also added comments to better describe the
references for these algorithms.  Although the revised expressions are all
mathematically equivalent, this commit will change answers for any cases that
use EQN_OF_STATE = "UNESCO".  However, it is believed based on a survey of the
MOM6 community that there are no active configurations that use this equation of
state.