Planner: Added meta data constant class to aid in retrieving the meta data by their key names.

Tools/ArdupilotMegaPlanner/Constants/ParameterMetaDataConstants.cs

+namespace ArdupilotMega.Constants
+{
+   public sealed class ParameterMetaDataConstants
+   {
+      public const string Delimeter = "@";
+      public const string Param = "Param";
+      public const string DisplayName = "DisplayName";
+      public const string Description = "Description";
+      public const string Units = "Units";
+      public const string Range = "Range";
+   }
+}