Tick generator: NumericManual should have a complimentary DateTimeManaul

[swharden]

Presently this works but it uses the "ToOADate" hack

ScottPlot.TickGenerators.NumericManual tickGenX = new();
tickGenX.AddMajor(myDateTime.ToOADate(), $"{dt1}");

[swharden]

An alternative quick fix could be to provide Add() overloads which accept DateTime

ScottPlot/src/ScottPlot5/ScottPlot5/TickGenerators/NumericManual.cs

Lines 41 to 49 in cdc226a

	public void AddMajor(double position, string label)
	{
	Add(Tick.Major(position, label));
	}
	public void AddMinor(double position)
	{
	Add(Tick.Minor(position));
	}

[swharden]

Related, Axes.SetLimits() should have overloads that accept DateTime