Add nzero index (closes #146)

climix/etc/smhi_indices.yml

+indices:
+  nzero:
+    reference: SMHI
+    period:
+      allowed:
+        annual:
+        monthly:
+        seasonal:
+      default: annual
+    output:
+      var_name: nzero
+      standard_name:
+      long_name: Number of zero-crossing days (days when Tmin < 0 degC < Tmax)
+      units: days
+      cell_methods:
+        - time: sum over days
+      proposed_cell_methods:
+        - time: level_crossing within days
+        - time: sum over days
+    input:
+      low_data: tasmin
+      high_data: tasmax
+    index_function:
+      name: count_level_crossings
+      parameters:
+        threshold:
+          kind: quantity
+          long_name: Threshold value for daily air temperature
+          standard_name: air_temperature
+          data: 0.
+          units: degree_Celsius