Fix broken inheritance from DataFrameColumn class

src/Microsoft.Data.Analysis/DataFrameColumn.cs

@@ -460,7 +460,7 @@ public string ToString(long rowsToShow)
         /// </summary>
         /// <param name="ascending">Sorting order.</param>
         /// <param name="putNullValuesLast">If true, null values are always put at the end.</param>
-        internal abstract PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast);
+        protected internal abstract PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast);
 
         protected delegate long GetBufferSortIndex(int bufferIndex, int sortIndex);
         protected delegate ValueTuple<T, int> GetValueAndBufferSortIndexAtBuffer<T>(int bufferIndex, int valueIndex);

src/Microsoft.Data.Analysis/DataFrameColumns/ArrowStringDataFrameColumn.cs

@@ -363,7 +363,7 @@ protected internal override Apache.Arrow.Array ToArrowArray(long startIndex, int
             return new StringArray(numberOfRows, offsetsBuffer, dataBuffer, nullBuffer, nullCount, indexInBuffer);
         }
 
-        internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast) => throw new NotSupportedException();
+        protected internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast) => throw new NotSupportedException();
 
         public new ArrowStringDataFrameColumn Clone(long numberOfNullsToAppend = 0)
         {

src/Microsoft.Data.Analysis/DataFrameColumns/StringDataFrameColumn.cs

@@ -202,7 +202,7 @@ public IEnumerator<string> GetEnumerator()
             return (StringDataFrameColumn)base.Sort(ascending, putNullValuesLast);
         }
 
-        internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast)
+        protected internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast)
         {
             var comparer = Comparer<string>.Default;
 

src/Microsoft.Data.Analysis/DataFrameColumns/VBufferDataFrameColumn.cs

@@ -384,6 +384,6 @@ protected override DataFrameColumn DropNullsImplementation()
             return Clone();
         }
 
-        internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast) => throw new NotImplementedException();
+        protected internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending, bool putNullValuesLast) => throw new NotImplementedException();
     }
 }

src/Microsoft.Data.Analysis/PrimitiveDataFrameColumn.Sort.cs

@@ -18,7 +18,7 @@ public partial class PrimitiveDataFrameColumn<T> : DataFrameColumn
             return (PrimitiveDataFrameColumn<T>)base.Sort(ascending, putNullValuesLast);
         }
 
-        internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending = true, bool putNullValuesLast = true)
+        protected internal override PrimitiveDataFrameColumn<long> GetSortIndices(bool ascending = true, bool putNullValuesLast = true)
         {
             var comparer = Comparer<T>.Default;