adding IXLTemplate interface (#269)

Co-authored-by: James Davis (Contractor) <james.davis8@gilead.com>

Author: davija

ClosedXML.Report/IXLTemplate.cs

@@ -0,0 +1,29 @@
+using System;
+using System.IO;
+using ClosedXML.Excel;
+
+namespace ClosedXML.Report
+{
+    public interface IXLTemplate : IDisposable
+    {
+        public IXLWorkbook Workbook { get; }
+
+        public XLGenerateResult Generate();
+
+        public void AddVariable(object value);
+
+        public void AddVariable(string alias, object value);
+
+        public void SaveAs(string file);
+
+        public void SaveAs(string file, SaveOptions options);
+
+        public void SaveAs(string file, bool validate, bool evaluateFormulae = false);
+
+        public void SaveAs(Stream stream);
+
+        public void SaveAs(Stream stream, SaveOptions options);
+
+        public void SaveAs(Stream stream, bool validate, bool evaluateFormulae = false);
+    }
+}

ClosedXML.Report/XLTemplate.cs

@@ -3,15 +3,14 @@
 using ClosedXML.Report.Options;
 using System;
 using System.Collections;
-using System.Collections.Generic;
 using System.Data;
 using System.IO;
 using System.Linq;
 using System.Reflection;
 
 namespace ClosedXML.Report
 {
-    public class XLTemplate : IDisposable
+    public class XLTemplate : IXLTemplate
     {
         private readonly RangeInterpreter _interpreter;
         private readonly bool _disposeWorkbookWithTemplate;