make improvements of the data reader

Author: xieguigang

Data/DataFrame/IO/DataFrame/HeaderSchema.vb

@@ -205,6 +205,7 @@ Namespace IO
                 .SeqIterator _
                 .ToDictionary(Function(i) i.value,
                               Function(i)
+                                  ' 0-based ordinal offset
                                   Return i.i
                               End Function)
         End Function

Microsoft.VisualBasic.Core/src/Extensions/Collection/Linq/SeqValue.vb

@@ -122,11 +122,21 @@ Namespace Linq
             Return $"[{i}] {Me.value.GetJson(False)}"
         End Function
 
+        ''' <summary>
+        ''' get value from the indexed object
+        ''' </summary>
+        ''' <param name="x"></param>
+        ''' <returns></returns>
         <MethodImpl(MethodImplOptions.AggressiveInlining)>
         Public Shared Narrowing Operator CType(x As SeqValue(Of T)) As T
             Return x.value
         End Operator
 
+        ''' <summary>
+        ''' get ordinal index offset from the given indexed object.
+        ''' </summary>
+        ''' <param name="x"></param>
+        ''' <returns></returns>
         <MethodImpl(MethodImplOptions.AggressiveInlining)>
         Public Shared Narrowing Operator CType(x As SeqValue(Of T)) As Integer
             Return x.i