Add some marine variables (#131)

## Description
We need some marine variables added to the ESM standard since we wish to use them in generic JEDI code for use with SOCA. I have attempted to follow the conventions used for the corresponding `air` variables.

## Issues
Variables needed for JCSDA-internal/vader#208

Author: svahl991

Metadata-standard-names.md

@@ -6,6 +6,7 @@
 * [coordinates](#coordinates)
 * [state_variables](#state_variables)
 * [land_surface](#land_surface)
+* [marine](#marine)
 * [diagnostics](#diagnostics)
 * [atmospheric_composition](#atmospheric_composition)
 * [atmospheric_composition: GOCART aerosols](#atmospheric_composition-gocart-aerosols)
@@ -544,6 +545,17 @@ Note that appending '_on_previous_timestep' to standard_names in this section yi
     * `real`: units = m3 m-3
 * `volume_fraction_of_liquid_water_in_soil_at_wilting_point`: volume fraction of water in liquid phase in soil at wilting point
     * `real`: units = m3 m-3
+## marine
+* `sea_water_potential_temperature`: sea water potential temperature
+    * `real`: units = K
+* `sea_water_depth`: The depth below the surface of the sea
+    * `real`: units = m
+* `sea_water_practical_salinity`: The practical salinity of sea water
+    * `real`: units = PSU
+* `sea_water_absolute_salinity`: The absolute salinity of sea water
+    * `real`: units = g kg-1
+* `sea_water_temperature`: The temperature of sea water
+    * `real`: units = K
 ## diagnostics
 * `total_precipitation_rate_at_surface`: Total precipitation rate at surface
     * `real`: units = m s-1

Metadata-standard-names.yaml

@@ -1264,6 +1264,34 @@ section:
     type: real
     kind: null
     units: m3 m-3
+- name: marine
+  comment: null
+  standard_names:
+  - name: sea_water_potential_temperature
+    description: sea water potential temperature
+    type: real
+    kind: null
+    units: K
+  - name: sea_water_depth
+    description: The depth below the surface of the sea
+    type: real
+    kind: null
+    units: m
+  - name: sea_water_practical_salinity
+    description: The practical salinity of sea water
+    type: real
+    kind: null
+    units: PSU
+  - name: sea_water_absolute_salinity
+    description: The absolute salinity of sea water
+    type: real
+    kind: null
+    units: g kg-1
+  - name: sea_water_temperature
+    description: The temperature of sea water
+    type: real
+    kind: null
+    units: K
 - name: diagnostics
   comment: null
   standard_names:

standard_names.xml

@@ -1000,6 +1000,28 @@
       <type units="m3 m-3">real</type>
     </standard_name>
   </section>
+  <section name="marine">
+    <standard_name name="sea_water_potential_temperature"
+                   description="sea water potential temperature">
+      <type units="K">real</type>
+    </standard_name>
+    <standard_name name="sea_water_depth"
+                     description="The depth below the surface of the sea">
+        <type units="m">real</type>
+    </standard_name>
+    <standard_name name="sea_water_practical_salinity"
+                   description="The practical salinity of sea water">
+       <type units="PSU">real</type>
+    </standard_name>
+    <standard_name name="sea_water_absolute_salinity"
+                   description="The absolute salinity of sea water">
+       <type units="g kg-1">real</type>
+    </standard_name>
+    <standard_name name="sea_water_temperature"
+                     description="The temperature of sea water">
+        <type units="K">real</type>
+    </standard_name>
+  </section>
   <section name="diagnostics">
     <standard_name name="total_precipitation_rate_at_surface">
       <type units="m s-1">real</type>