*Refactor MOM_EOS_NEMO.F90

  Refactored the expressions in MOM_EOS_NEMO.F90, adding parentheses to specify
the order of arithmetic, starting with the highest-order terms first for less
sensitivity to round-off.  A number of internal variables were also renamed for
greater clarity, and a number of comments were revised 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 = "NEMO".  However, there is another recent commit to this file
that also changes answers (specifically the density derivatives) with this
equation of state, and it is believed based on a survey of the MOM6 community
that there are no active configurations that use this equation of state.