Refactor and optimize TBar, TBarSeries, and TSeries notebooks; remove obsolete code

- Enhanced Alma class by simplifying the CalculateWeightedSum method and removing unnecessary comments.
- Removed SIMD-related methods from Conv class, replacing them with optimized DotProduct calls.
- Updated Sma and Wma classes to use source.ContainsNonFinite() for non-finite value checks, improving readability and performance.

lib/core/ringbuffer/RingBuffer.cs

@@ -136,6 +136,15 @@ public sealed class RingBuffer : IEnumerable<double>
         }
     }
 
+    /// <summary>
+    /// Gets the index in the internal buffer where the oldest element is located.
+    /// </summary>
+    public int StartIndex
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        get => _count == _capacity ? _head : 0;
+    }
+
     /// <summary>
     /// Gets a read-only span over the internal buffer array for direct SIMD access.
     /// </summary>