Add Negative Volume Index (NVI) implementation and tests

- Implemented NVI indicator in Nvi.Quantower.cs with configurable start value and cold value display option.
- Created unit tests for NVI functionality in Nvi.Tests.cs, covering various scenarios including initialization, updates, and edge cases.
- Added validation tests in Nvi.Validation.Tests.cs to ensure NVI matches expected behavior against known implementations.
- Developed comprehensive documentation for NVI in Nvi.md, detailing its historical context, mathematical foundation, and interpretation guide.
- Included error handling for invalid input values and ensured compatibility with volume data.

lib/volume/mfi/Mfi.Quantower.cs

@@ -0,0 +1,51 @@
+using System.Drawing;
+using System.Runtime.CompilerServices;
+using TradingPlatform.BusinessLayer;
+
+namespace QuanTAlib;
+
+[SkipLocalsInit]
+public sealed class MfiIndicator : Indicator, IWatchlistIndicator
+{
+    [InputParameter("Period", sortIndex: 10, 1, 500, 1, 0)]
+    public int Period { get; set; } = 14;
+
+    [InputParameter("Show cold values", sortIndex: 21)]
+    public bool ShowColdValues { get; set; } = true;
+
+    private Mfi _mfi = null!;
+    private readonly LineSeries _series;
+
+    public int MinHistoryDepths => Period;
+    int IWatchlistIndicator.MinHistoryDepths => Period;
+
+    public override string ShortName => $"MFI({Period})";
+    public override string SourceCodeLink => "https://github.com/mihakralj/QuanTAlib/blob/main/lib/volume/mfi/Mfi.Quantower.cs";
+
+    public MfiIndicator()
+    {
+        OnBackGround = true;
+        SeparateWindow = true;
+        Name = "MFI - Money Flow Index";
+        Description = "Money Flow Index is a volume-weighted RSI that measures buying and selling pressure";
+
+        _series = new LineSeries(name: "MFI", color: Color.Blue, width: 2, style: LineStyle.Solid);
+        AddLineSeries(_series);
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected override void OnInit()
+    {
+        _mfi = new Mfi(Period);
+        base.OnInit();
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    protected override void OnUpdate(UpdateArgs args)
+    {
+        TBar bar = this.GetInputBar(args);
+        TValue result = _mfi.Update(bar, args.IsNewBar());
+
+        _series.SetValue(result.Value, _mfi.IsHot, ShowColdValues);
+    }
+}