Added DbForeignObject for automatic resolving

Added Init() to initialise necessary classes
Changed structure and added a BaseAttribute
Fixed occuring errors to match new system

Database-Attribute_System/Attributes/BaseAttributes/DbPrimaryKey.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Threading.Tasks;
+
+namespace eu.railduction.netcore.dll.Database_Attribute_System.Attributes
+{
+    [AttributeUsage(AttributeTargets.Field, Inherited = false, AllowMultiple = false)]
+    public class DbPrimaryKey : BaseAttribute
+    {
+        /// <summary>
+        /// Marks variable as primaryKey fo current class
+        /// </summary>
+        /// <param name="dbAttributeName">Name of database-attribute (case-sensitivity is determined from database-attribute-settings) ['null' if the same as field-name]</param>
+        public DbPrimaryKey(string attributeName = null)
+        {
+            this._attributeName = attributeName;    // Todo: Automatic resolving of name if it is null (?)
+        }
+
+        public void Init(FieldInfo fi, DbObject classAttribute)
+        {
+            this.parentField = fi;
+            this.classAttribute = classAttribute;
+
+            this._attributeName = this._attributeName ?? fi.Name;     // If no alternative attribute-name is specified, use the property-name
+        }
+    }
+}