aw6374: each year new calculation model; fix OM -> Carbon switch, calculation based on carbon

Models/FarmMapsCarbonRequest.cs

@@ -13,6 +13,7 @@ namespace FarmMapsAPI.Carbon.Models
 {
     public class FarmMapsCarbonRequest
     {
+        public int? ModelYear { get; set; }
         public string ScenarioName { get; set; }
         public int PostalCode { get; set; }
 
@@ -76,6 +77,7 @@ namespace FarmMapsAPI.Carbon.Models
     public class SoilPropertyFarmMaps
     {
         public double? OM_Const { get; set; }
+        public double? C_Const { get; set; }
         public string OM_Choice { get; set; }
         public double? Clay_Content { get; set; }
         public bool Depth10 { get; set; }