Update DB to version 20: Machines can have an arbitrary number of storage,

so use an interconnection table, `storage_by_machine`.

Cicm.Database/Operations/Operations.cs

@@ -35,7 +35,7 @@ namespace Cicm.Database
     public partial class Operations
     {
         /// <summary>Last known database version</summary>
-        const int DB_VERSION = 19;
+        const int DB_VERSION = 20;
         /// <summary>The column with this value indicates there is no item of this type.</summary>
         public const int DB_NONE = -1;
         /// <summary>