Add logic for import of combination from xls files

StructureHelperCommon/Models/Forces/FilePropertyFactory.cs

@@ -0,0 +1,40 @@
+using StructureHelperCommon.Infrastructures.Exceptions;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperCommon.Models.Forces
+{
+    public enum FilePropertyType
+    {
+        Forces
+    }
+    public static class FilePropertyFactory
+    {
+        public static IColumnedFileProperty GetForceFileProperty(FilePropertyType propertyType)
+        {
+            if (propertyType == FilePropertyType.Forces)
+            {
+                ColumnedFileProperty fileProperty = new();
+                List<IColumnProperty> columnProperties = GetForceColumns();
+                fileProperty.ColumnProperties.AddRange(columnProperties);
+                return fileProperty;
+            }
+            else
+            {
+                throw new StructureHelperException(ErrorStrings.ObjectTypeIsUnknownObj(propertyType));
+            }
+        }
+
+        private static List<IColumnProperty> GetForceColumns()
+        {
+            List<IColumnProperty> columnProperties = new();
+            columnProperties.Add(new ColumnProperty("Nz") { SearchingName = "N", Index = 6 });
+            columnProperties.Add(new ColumnProperty("Mx") { SearchingName = "My", Index = 8 });
+            columnProperties.Add(new ColumnProperty("My") { SearchingName = "Mz", Index = 10 });
+            return columnProperties;
+        }
+    }
+}