Implementing Error bars

[KrisWragg]

Created ExcelChartErrorBar class that implements all functionality documented.
Added ExcelChartErrorBar to:

• ExcelBarChartSerie as ErrorBar
• ExcelScatterChartSerie as HorizontalErrorBar and VerticalErrorBar
• ExcelLineChartSerie as ErrorBar

Automatically adds error bars when first accessed, Delete function available to remove each error bar if necessary.

Also fixed a bug in ExcelChartSeries which failed to recreate the series as type ExcelBarChartSerie when loading a bar chart in. Found when testing my example.

Created Sample_ErrorBars that shows the functionality in action for the three chart types.

Would be great to get this merged in and into a new version as I require this functionality for a project I am working on and is missing from the latest NuGet package.

[rummelsworth]

Unless I'm missing something, any of these 3 assignments will overwrite whatever value is retrieved by GetXmlNodeDoubleNull.

Consider rewriting this setter like this or similar:

double defaultValue;
switch (ValueType)
{
    case eErrorBarValueType.CustomErrorBars:
        throw new Exception("Error bar value is not valid for Custom Error Bars, use PlusAddress and MinusAddress");
    case eErrorBarValueType.StandardError:
        throw new Exception("Error bar value is not valid for Standard Error");
    case eErrorBarValueType.FixedValue:
        defaultValue = 0.1;
        break;
    case eErrorBarValueType.Percentage:
        defaultValue = 5;
        break;
    case eErrorBarValueType.StandardDeviation:
        defaultValue = 1.0;
        break;
}

return GetXmlNodeDoubleNull(_errorBarValuePath) ?? defaultValue;