Add ch4 surface custom cmor table entry (#2168)

Co-authored-by: Rémi Kazeroni <remi.kazeroni@dlr.de>

esmvalcore/cmor/tables/custom/CMOR_ch4s.dat

@@ -0,0 +1,22 @@
+SOURCE: CMIP5
+!============
+variable_entry:    ch4s
+!============
+modeling_realm:    atmos
+!----------------------------------
+! Variable attributes:
+!----------------------------------
+standard_name:     mole_fraction_of_methane_in_air
+units:             1e-09
+cell_methods:      area: time: mean
+cell_measures:     area: areacella
+long_name:         Atmosphere CH4 surface
+comment:           As ch4, but only at the surface
+!----------------------------------
+! Additional variable information:
+!----------------------------------
+dimensions:        longitude latitude time
+out_name:          ch4s
+type:              real
+!----------------------------------
+!

tests/integration/cmor/test_table.py

@@ -476,3 +476,12 @@ def test_get_variable_tasaga(self):
         self.assertEqual(var.long_name,
                          'Global-mean Near-Surface Air Temperature Anomaly')
         self.assertEqual(var.units, 'K')
+
+    def test_get_variable_ch4s(self):
+        """Get tas variable."""
+        CustomInfo()
+        var = self.variables_info.get_variable('Amon', 'ch4s')
+        self.assertEqual(var.short_name, 'ch4s')
+        self.assertEqual(var.long_name,
+                         'Atmosphere CH4 surface')
+        self.assertEqual(var.units, '1e-09')