+Add EOS_unit_tests

  Added the new publicly visible function EOS_unit_tests, along with a call to
it from inside of unit_tests.  These tests evaluate check values for density and
assess the consistency of expressions for variables that can be derived from
density with finite-difference estimates of the same variables.  These tests
reveal inconsistencies or omissions with several of the options for the equation
of state.  The EOS self-consistency tests that are failing are commented out for
now, so that this redacted unit test passes.  All answers are bitwise identical,
but there can be new diagnostic messages written out.

src/core/MOM_unit_tests.F90

@@ -11,6 +11,7 @@ module MOM_unit_tests
 use MOM_random,                     only : random_unit_tests
 use MOM_lateral_boundary_diffusion, only : near_boundary_unit_tests
 use MOM_CFC_cap,                    only : CFC_cap_unit_tests
+use MOM_EOS,                        only : EOS_unit_tests
 implicit none ; private
 
 public unit_tests
@@ -30,6 +31,8 @@ subroutine unit_tests(verbosity)
   if (is_root_pe()) then ! The following need only be tested on 1 PE
     if (string_functions_unit_tests(verbose)) call MOM_error(FATAL, &
        "MOM_unit_tests: string_functions_unit_tests FAILED")
+    if (EOS_unit_tests(verbose)) call MOM_error(FATAL, &
+       "MOM_unit_tests: EOS_unit_tests FAILED")
     if (remapping_unit_tests(verbose)) call MOM_error(FATAL, &
        "MOM_unit_tests: remapping_unit_tests FAILED")
     if (neutral_diffusion_unit_tests(verbose)) call MOM_error(FATAL, &